diff --git a/xo-expression2/CMakeLists.txt b/xo-expression2/CMakeLists.txt
index 52e9d380..869f891d 100644
--- a/xo-expression2/CMakeLists.txt
+++ b/xo-expression2/CMakeLists.txt
@@ -124,6 +124,32 @@ xo_add_genfacetimpl(
 
 # ----------------------------------------------------------------
 
+# note: manual target; generated code committed to git
+xo_add_genfacetimpl(
+    TARGET xo-expression2-facetimpl-expression-applyexpr
+    FACET_PKG xo_expression2
+    FACET Expression
+    REPR ApplyExpr
+    INPUT idl/IExpression_DApplyExpr.json5
+    OUTPUT_HPP_DIR include/xo/expression2
+    OUTPUT_IMPL_SUBDIR detail
+    OUTPUT_CPP_DIR src/expression2
+)
+
+# note: manual target; generated code committed to git
+xo_add_genfacetimpl(
+    TARGET xo-expression2-facetimpl-printable-applyexpr
+    FACET_PKG xo_printable2
+    FACET Printable
+    REPR ApplyExpr
+    INPUT idl/IPrintable_DApplyExpr.json5
+    OUTPUT_HPP_DIR include/xo/expression2
+    OUTPUT_IMPL_SUBDIR detail
+    OUTPUT_CPP_DIR src/expression2
+)
+
+# ----------------------------------------------------------------
+
 # note: manual target; generated code committed to git
 xo_add_genfacetimpl(
     TARGET xo-expression2-facetimpl-gcobject-uniquestring
diff --git a/xo-expression2/idl/IExpression_DApplyExpr.json5 b/xo-expression2/idl/IExpression_DApplyExpr.json5
new file mode 100644
index 00000000..6814547c
--- /dev/null
+++ b/xo-expression2/idl/IExpression_DApplyExpr.json5
@@ -0,0 +1,12 @@
+{
+    mode: "implementation",
+    includes: [ "\"Expression.hpp\"" ],
+    local_types: [ ],
+    namespace1: "xo",
+    namespace2: "scm",
+    facet_idl: "idl/Expression.json5",
+    brief: "provide AExpression interface for DApplyExpr state",
+    using_doxygen: true,
+    repr: "DApplyExpr",
+    doc: ["doc for IExpression+DApplyExpr" ],
+}
diff --git a/xo-expression2/idl/IPrintable_DApplyExpr.json5 b/xo-expression2/idl/IPrintable_DApplyExpr.json5
new file mode 100644
index 00000000..fe743b0b
--- /dev/null
+++ b/xo-expression2/idl/IPrintable_DApplyExpr.json5
@@ -0,0 +1,13 @@
+{
+    mode: "implementation",
+    includes: [ "<xo/printable2/Printable.hpp>",
+                "<xo/printable2/detail/IPrintable_Xfer.hpp>" ],
+    local_types: [ ],
+    namespace1: "xo",
+    namespace2: "scm",
+    facet_idl: "idl/Printable.json5",
+    brief: "provide APrintable interface for DApplyExpr",
+    using_doxygen: true,
+    repr: "DApplyExpr",
+    doc: [ "implement APrintable for DApplyExpr" ],
+}
diff --git a/xo-expression2/include/xo/expression2/DApplyExpr.hpp b/xo-expression2/include/xo/expression2/DApplyExpr.hpp
index 882cf5cd..8502c62a 100644
--- a/xo-expression2/include/xo/expression2/DApplyExpr.hpp
+++ b/xo-expression2/include/xo/expression2/DApplyExpr.hpp
@@ -20,18 +20,55 @@ namespace xo {
          **/
         class DApplyExpr {
         public:
+            using AAllocator = xo::mm::AAllocator;
             using TypeDescr = xo::reflect::TypeDescr;
             using ppindentinfo = xo::print::ppindentinfo;
-            using size_type = std::size_t;
+            using size_type = std::uint32_t;
 
         public:
+            /** @defgroup scm-applyexpr-constructors **/
+            ///@{
+
+            /** construct empty instance, but with argument expressions empty **/
+            DApplyExpr(TypeRef typeref,
+                       obj<AExpression> fn_expr,
+                       size_type n_args);
+
+            /** create apply for function with 2 arguments **/
+            static obj<AExpression,DApplyExpr> make2(obj<AAllocator> mm,
+                                                     TypeRef typeref,
+                                                     obj<AExpression> fn_expr,
+                                                     obj<AExpression> arg1,
+                                                     obj<AExpression> arg2);
+
+            /** create apply for function with 2 arguments **/
+            static DApplyExpr * _make2(obj<AAllocator> mm,
+                                       TypeRef typeref,
+                                       obj<AExpression> fn_expr,
+                                       obj<AExpression> arg1,
+                                       obj<AExpression> arg2);
+
+            /** scaffold incomplete instance.
+             *  apply-expr using memory from @p mm.
+             *  will construct instance with space for @p n_args arguments
+             *  but expressions left empty.
+             *  use @ref assign_arg for all arguments to complete.
+             **/
+            static DApplyExpr * scaffold(obj<AAllocator> mm,
+                                         TypeRef typeref,
+                                         obj<AExpression> fn_expr,
+                                         size_type n_args);
+            void assign_arg(size_type i_arg, obj<AExpression> expr);
+
+            ///@}
+            /** @defgroup scm-applyexpr-access-methods **/
+            ///@{
 
             obj<AExpression>  fn() const noexcept { return fn_; }
-            const DArray * args() const noexcept { return args_; }
-
-            size_type n_arg() const noexcept { return args_->size(); }
+            size_type n_args() const noexcept { return n_args_; }
             obj<AExpression> arg(size_type i) const;
 
+            ///@}
             /** @defgroup scm-applyexpr-expression-facet **/
             ///@{
 
@@ -40,6 +77,12 @@ namespace xo {
             TypeDescr valuetype() const noexcept { return typeref_.td(); }
             void assign_valuetype(TypeDescr td) noexcept;
 
+            ///@}
+            /** @defgroup scm-applyexpr-gcobject-facet **/
+            ///@{
+
+            // shallow_copy() etc.
+
             ///@}
             /** @defgroup scm-applyexpr-printable-facet **/
             ///@{
@@ -55,8 +98,10 @@ namespace xo {
             TypeRef typeref_;
             /** expression for function/procedure to invoke **/
             obj<AExpression> fn_;
-            /** expression for each argument vector **/
-            const DArray * args_;
+            /** number of arguments (not counting @ref fn_ **/
+            size_type n_args_ = 0;
+            /** args_[i] is expression for i'th argument to @ref fn_ **/
+            obj<AExpression> args_[];
         };
     }
 }
diff --git a/xo-expression2/include/xo/expression2/detail/IExpression_DApplyExpr.hpp b/xo-expression2/include/xo/expression2/detail/IExpression_DApplyExpr.hpp
new file mode 100644
index 00000000..c8952199
--- /dev/null
+++ b/xo-expression2/include/xo/expression2/detail/IExpression_DApplyExpr.hpp
@@ -0,0 +1,66 @@
+/** @file IExpression_DApplyExpr.hpp
+ *
+ *  Generated automagically from ingredients:
+ *  1. code generator:
+ *       [/home/roland/proj/xo-umbrella2-claude1/xo-facet/codegen/genfacet]
+ *     arguments:
+ *       --input [idl/IExpression_DApplyExpr.json5]
+ *  2. jinja2 template for abstract facet .hpp file:
+ *       [iface_facet_repr.hpp.j2]
+ *  3. idl for facet methods
+ *       [idl/IExpression_DApplyExpr.json5]
+ **/
+
+#pragma once
+
+#include "Expression.hpp"
+#include "Expression.hpp"
+#include "DApplyExpr.hpp"
+
+namespace xo { namespace scm { class IExpression_DApplyExpr; } }
+
+namespace xo {
+    namespace facet {
+        template <>
+        struct FacetImplementation<xo::scm::AExpression,
+                                   xo::scm::DApplyExpr>
+        {
+            using ImplType = xo::scm::IExpression_Xfer
+              <xo::scm::DApplyExpr,
+               xo::scm::IExpression_DApplyExpr>;
+        };
+    }
+}
+
+namespace xo {
+    namespace scm {
+        /** @class IExpression_DApplyExpr
+         **/
+        class IExpression_DApplyExpr {
+        public:
+            /** @defgroup scm-expression-dapplyexpr-type-traits **/
+            ///@{
+            using TypeDescr = xo::scm::AExpression::TypeDescr;
+            using Copaque = xo::scm::AExpression::Copaque;
+            using Opaque = xo::scm::AExpression::Opaque;
+            ///@}
+            /** @defgroup scm-expression-dapplyexpr-methods **/
+            ///@{
+            // const methods
+            /** expression type (constant | apply | ..) **/
+            static exprtype extype(const DApplyExpr & self) noexcept;
+            /** placeholder for type giving possible values for this expression **/
+            static TypeRef typeref(const DApplyExpr & self) noexcept;
+            /** type giving possible values for this expression. Maybe null before typecheck **/
+            static TypeDescr valuetype(const DApplyExpr & self) noexcept;
+
+            // non-const methods
+            /** assing to valuetype member. Useful when scaffolding expressions **/
+            static void assign_valuetype(DApplyExpr & self, TypeDescr td) noexcept;
+            ///@}
+        };
+
+    } /*namespace scm*/
+} /*namespace xo*/
+
+/* end */
\ No newline at end of file
diff --git a/xo-expression2/include/xo/expression2/detail/IPrintable_DApplyExpr.hpp b/xo-expression2/include/xo/expression2/detail/IPrintable_DApplyExpr.hpp
new file mode 100644
index 00000000..c05e87e2
--- /dev/null
+++ b/xo-expression2/include/xo/expression2/detail/IPrintable_DApplyExpr.hpp
@@ -0,0 +1,62 @@
+/** @file IPrintable_DApplyExpr.hpp
+ *
+ *  Generated automagically from ingredients:
+ *  1. code generator:
+ *       [/home/roland/proj/xo-umbrella2-claude1/xo-facet/codegen/genfacet]
+ *     arguments:
+ *       --input [idl/IPrintable_DApplyExpr.json5]
+ *  2. jinja2 template for abstract facet .hpp file:
+ *       [iface_facet_repr.hpp.j2]
+ *  3. idl for facet methods
+ *       [idl/IPrintable_DApplyExpr.json5]
+ **/
+
+#pragma once
+
+#include "Printable.hpp"
+#include <xo/printable2/Printable.hpp>
+#include <xo/printable2/detail/IPrintable_Xfer.hpp>
+#include "DApplyExpr.hpp"
+
+namespace xo { namespace scm { class IPrintable_DApplyExpr; } }
+
+namespace xo {
+    namespace facet {
+        template <>
+        struct FacetImplementation<xo::print::APrintable,
+                                   xo::scm::DApplyExpr>
+        {
+            using ImplType = xo::print::IPrintable_Xfer
+              <xo::scm::DApplyExpr,
+               xo::scm::IPrintable_DApplyExpr>;
+        };
+    }
+}
+
+namespace xo {
+    namespace scm {
+        /** @class IPrintable_DApplyExpr
+         **/
+        class IPrintable_DApplyExpr {
+        public:
+            /** @defgroup scm-printable-dapplyexpr-type-traits **/
+            ///@{
+            using ppindentinfo = xo::print::APrintable::ppindentinfo;
+            using Copaque = xo::print::APrintable::Copaque;
+            using Opaque = xo::print::APrintable::Opaque;
+            ///@}
+            /** @defgroup scm-printable-dapplyexpr-methods **/
+            ///@{
+            // const methods
+            /** Pretty-printing support for this object.
+See [xo-indentlog/xo/indentlog/pretty.hpp] **/
+            static bool pretty(const DApplyExpr & self, const ppindentinfo & ppii);
+
+            // non-const methods
+            ///@}
+        };
+
+    } /*namespace scm*/
+} /*namespace xo*/
+
+/* end */
\ No newline at end of file
diff --git a/xo-expression2/src/expression2/CMakeLists.txt b/xo-expression2/src/expression2/CMakeLists.txt
index 9dea6457..b8be9468 100644
--- a/xo-expression2/src/expression2/CMakeLists.txt
+++ b/xo-expression2/src/expression2/CMakeLists.txt
@@ -22,6 +22,9 @@ set(SELF_SRCS
     IExpression_DDefineExpr.cpp
     IPrintable_DDefineExpr.cpp
 
+    IExpression_DApplyExpr.cpp
+    IPrintable_DApplyExpr.cpp
+
     DLocalSymtab.cpp
     DGlobalSymtab.cpp
 
diff --git a/xo-expression2/src/expression2/DApplyExpr.cpp b/xo-expression2/src/expression2/DApplyExpr.cpp
index 0c0546f3..ce5e1951 100644
--- a/xo-expression2/src/expression2/DApplyExpr.cpp
+++ b/xo-expression2/src/expression2/DApplyExpr.cpp
@@ -11,30 +11,62 @@
 namespace xo {
     using xo::print::APrintable;
     using xo::facet::FacetRegistry;
+    using xo::reflect::typeseq;
     using xo::mm::AGCObject;
 
     namespace scm {
+        /* incomplete! */
+        DApplyExpr::DApplyExpr(TypeRef typeref,
+                               obj<AExpression> fn_expr,
+                               size_type n_args) : typeref_{typeref},
+                                                   fn_{fn_expr},
+                                                   n_args_{n_args}
+        {}
+
+        DApplyExpr *
+        DApplyExpr::scaffold(obj<AAllocator> mm,
+                             TypeRef typeref,
+                             obj<AExpression> fn_expr,
+                             size_type n_args)
+        {
+            void * mem = mm.alloc(typeseq::id<DApplyExpr>(),
+                                  sizeof(DApplyExpr) + (n_args * sizeof(obj<AExpression>)));
+
+            DApplyExpr * result = new (mem) DApplyExpr(typeref,
+                                                       fn_expr,
+                                                       n_args);
+
+            return result;
+        }
+
+        void
+        DApplyExpr::assign_arg(size_type i_arg,
+                               obj<AExpression> expr)
+        {
+            if (i_arg < n_args_) {
+                this->args_[i_arg] = expr;
+            } else {
+                assert(false);
+
+                throw std::runtime_error(tostr("assign out-of-range argument i_arg where [0..n_args) expected",
+                                               xtag("i_arg", i_arg),
+                                               xtag("expr", expr),
+                                               xtag("n_args", n_args_)));
+
+            }
+        }
+
         obj<AExpression>
         DApplyExpr::arg(size_type i) const
         {
-            if (i >= args_->size()) [[unlikely]] {
+            if (i >= n_args_) [[unlikely]] {
                 throw std::runtime_error(tostr("attempt to fetch argument i where [0..n) expected",
                                                xtag("i", i),
-                                               xtag("n", args_->size()),
+                                               xtag("n", n_args_),
                                                xtag("src", "DApplyExpr::arg")));
             }
 
-            obj<AGCObject> arg_i = args_->at(i);
-
-            auto expr_i = FacetRegistry::instance().variant<AExpression>(arg_i);
-
-            if (!expr_i) [[unlikely]] {
-                throw std::runtime_error(tostr("expected expression interface on argument i",
-                                               xtag("i", i),
-                                               xtag("arg[i]", arg_i)));
-            }
-
-            return expr_i;
+            return args_[i];
         }
 
         void
@@ -60,9 +92,9 @@ namespace xo {
                         return false;
                 }
 
-                for (size_t i_arg = 0, n_arg = this->n_arg(); i_arg < n_arg; ++i_arg) {
+                for (size_t i_arg = 0; i_arg < n_args_; ++i_arg) {
                     obj<APrintable> arg_i
-                        = FacetRegistry::instance().variant<APrintable>(this->arg(i_arg));
+                        = FacetRegistry::instance().variant<APrintable>(args_[i_arg]);
 
                     if (!pps->print_upto(refrtag(concat("arg", 1+i_arg), arg_i)))
                         return false;
@@ -78,13 +110,14 @@ namespace xo {
                 pps->newline_indent(ppii.ci1());
                 pps->pretty(refrtag("fn", fn));
 
-                for (size_t i_arg = 0, n_arg = this->n_arg(); i_arg < n_arg; ++i_arg) {
+                for (size_t i_arg = 0; i_arg < n_args_; ++i_arg) {
                     obj<APrintable> arg_i
-                        = FacetRegistry::instance().variant<APrintable>(fn_);
+                        = FacetRegistry::instance().variant<APrintable>(args_[i_arg]);
 
                     pps->newline_indent(ppii.ci1());
                     pps->pretty(refrtag(concat("arg", 1+i_arg), arg_i));
                 }
+
                 return false;
             }
         }
diff --git a/xo-expression2/src/expression2/IExpression_DApplyExpr.cpp b/xo-expression2/src/expression2/IExpression_DApplyExpr.cpp
new file mode 100644
index 00000000..2098e050
--- /dev/null
+++ b/xo-expression2/src/expression2/IExpression_DApplyExpr.cpp
@@ -0,0 +1,45 @@
+/** @file IExpression_DApplyExpr.cpp
+ *
+ *  Generated automagically from ingredients:
+ *  1. code generator:
+ *       [/home/roland/proj/xo-umbrella2-claude1/xo-facet/codegen/genfacet]
+ *     arguments:
+ *       --input [idl/IExpression_DApplyExpr.json5]
+ *  2. jinja2 template for abstract facet .hpp file:
+ *       [iface_facet_any.hpp.j2]
+ *  3. idl for facet methods
+ *       [idl/IExpression_DApplyExpr.json5]
+**/
+
+#include "detail/IExpression_DApplyExpr.hpp"
+
+namespace xo {
+    namespace scm {
+        auto
+        IExpression_DApplyExpr::extype(const DApplyExpr & self) noexcept -> exprtype
+        {
+            return self.extype();
+        }
+
+        auto
+        IExpression_DApplyExpr::typeref(const DApplyExpr & self) noexcept -> TypeRef
+        {
+            return self.typeref();
+        }
+
+        auto
+        IExpression_DApplyExpr::valuetype(const DApplyExpr & self) noexcept -> TypeDescr
+        {
+            return self.valuetype();
+        }
+
+        auto
+        IExpression_DApplyExpr::assign_valuetype(DApplyExpr & self, TypeDescr td) noexcept -> void
+        {
+            self.assign_valuetype(td);
+        }
+
+    } /*namespace scm*/
+} /*namespace xo*/
+
+/* end IExpression_DApplyExpr.cpp */
\ No newline at end of file
diff --git a/xo-expression2/src/expression2/IPrintable_DApplyExpr.cpp b/xo-expression2/src/expression2/IPrintable_DApplyExpr.cpp
new file mode 100644
index 00000000..4f4a50d3
--- /dev/null
+++ b/xo-expression2/src/expression2/IPrintable_DApplyExpr.cpp
@@ -0,0 +1,28 @@
+/** @file IPrintable_DApplyExpr.cpp
+ *
+ *  Generated automagically from ingredients:
+ *  1. code generator:
+ *       [/home/roland/proj/xo-umbrella2-claude1/xo-facet/codegen/genfacet]
+ *     arguments:
+ *       --input [idl/IPrintable_DApplyExpr.json5]
+ *  2. jinja2 template for abstract facet .hpp file:
+ *       [iface_facet_any.hpp.j2]
+ *  3. idl for facet methods
+ *       [idl/IPrintable_DApplyExpr.json5]
+**/
+
+#include "detail/IPrintable_DApplyExpr.hpp"
+
+namespace xo {
+    namespace scm {
+        auto
+        IPrintable_DApplyExpr::pretty(const DApplyExpr & self, const ppindentinfo & ppii) -> bool
+        {
+            return self.pretty(ppii);
+        }
+
+
+    } /*namespace scm*/
+} /*namespace xo*/
+
+/* end IPrintable_DApplyExpr.cpp */
\ No newline at end of file
diff --git a/xo-expression2/src/expression2/expression2_register_facets.cpp b/xo-expression2/src/expression2/expression2_register_facets.cpp
index 1854f3eb..fc6ef3c1 100644
--- a/xo-expression2/src/expression2/expression2_register_facets.cpp
+++ b/xo-expression2/src/expression2/expression2_register_facets.cpp
@@ -17,6 +17,9 @@
 #include <xo/expression2/detail/IExpression_DConstant.hpp>
 #include <xo/expression2/detail/IPrintable_DConstant.hpp>
 
+#include <xo/expression2/detail/IExpression_DApplyExpr.hpp>
+#include <xo/expression2/detail/IPrintable_DApplyExpr.hpp>
+
 #include <xo/gc/detail/AGCObject.hpp>
 #include <xo/printable2/detail/APrintable.hpp>
 #include <xo/facet/FacetRegistry.hpp>
@@ -40,7 +43,8 @@ namespace xo {
             // Expression
             // +- Constant
             // +- Variable
-            // \- DefineExpr
+            // +- DefineExpr
+            // \- ApplyExpr
 
             FacetRegistry::register_impl<AExpression, DConstant>();
             FacetRegistry::register_impl<APrintable, DConstant>();
@@ -51,10 +55,14 @@ namespace xo {
             FacetRegistry::register_impl<AExpression, DDefineExpr>();
             FacetRegistry::register_impl<APrintable, DDefineExpr>();
 
+            FacetRegistry::register_impl<AExpression, DApplyExpr>();
+            FacetRegistry::register_impl<APrintable, DApplyExpr>();
+
             log && log(xtag("DUniqueString.tseq", typeseq::id<DUniqueString>()));
             log && log(xtag("DDefineExpr.tseq", typeseq::id<DDefineExpr>()));
             log && log(xtag("DVariable.tseq", typeseq::id<DVariable>()));
             log && log(xtag("DConstant.tseq", typeseq::id<DConstant>()));
+            log && log(xtag("DApplyExpr.tseq", typeseq::id<DApplyExpr>()));
 
             log && log(xtag("AExpression.tqseq", typeseq::id<AExpression>()));
 
diff --git a/xo-procedure2/idl/Procedure.json5 b/xo-procedure2/idl/Procedure.json5
index f26a4e56..91c8bdf3 100644
--- a/xo-procedure2/idl/Procedure.json5
+++ b/xo-procedure2/idl/Procedure.json5
@@ -1,3 +1,7 @@
+// can regenerate downstream .*pp files with either:
+//   cmake --build -- xo-procedure2-facet-procedure
+//   cmake --build -- xo-procedure2-genfacet-all
+
 {
     mode: "facet",
     // includes in ASyntaxStateMachine.hpp
@@ -56,6 +60,7 @@
             doc: ["invoke procedure; assume arguments satisfy type system" ],
             return_type: "obj<AGCObject>",
             args: [
+                {type: "obj<ARuntimeContext>", name: "rcx"},
                 {type: "const DArray *", name: "args"},
             ]
         }
diff --git a/xo-procedure2/include/xo/procedure2/DPrimitive.hpp b/xo-procedure2/include/xo/procedure2/DPrimitive.hpp
index cf913a15..afd2ada2 100644
--- a/xo-procedure2/include/xo/procedure2/DPrimitive.hpp
+++ b/xo-procedure2/include/xo/procedure2/DPrimitive.hpp
@@ -19,43 +19,49 @@
 
 namespace xo {
     namespace scm {
-        /** @brief Extract return type and argument types from a function type.
-         *
-         *  Primary template (undefined) - specializations handle specific cases
+        namespace detail {
+            /** @brief Extract return type and argument types from a function type.
+             *
+             *  Primary template (undefined) - specializations handle specific cases
+             **/
+            template <typename Fn>
+            struct PmFnTraits;
+
+            /** specialization for function pointers **/
+            template <typename R, typename... Args>
+            struct PmFnTraits<R(*)(obj<ARuntimeContext> rcx, Args...)> {
+                /** function return type **/
+                using return_type = R;
+                /** tuple type for function arguments (except for runtime context) **/
+                using args_tuple = std::tuple<Args...>;
+                /** number of arguments (except for runtime context) **/
+                static constexpr std::size_t n_args = sizeof...(Args);
+
+                /** arg_type<i> is the type of the i'th argument to Fn.
+                 *  (starting from argument after runtime context)
+                 **/
+                template <std::size_t I>
+                using arg_type = std::tuple_element_t<I, args_tuple>;
+            };
+
+            /** specialization for function references **/
+            template <typename R, typename... Args>
+            struct PmFnTraits<R(&)(obj<ARuntimeContext>, Args...)> : PmFnTraits<R(*)(obj<ARuntimeContext>, Args...)> {};
+
+            /** specialization for plain function types **/
+            template <typename R, typename... Args>
+            struct PmFnTraits<R(obj<ARuntimeContext>, Args...)> : PmFnTraits<R(*)(obj<ARuntimeContext>, Args...)> {};
+        }
+
+        /** @brief Schematika primitive with fixed number of arguments
          **/
-        template <typename Fn>
-        struct FnTraits;
-
-        /** specialization for function pointers **/
-        template <typename R, typename... Args>
-        struct FnTraits<R(*)(Args...)> {
-            /** function return type **/
-            using return_type = R;
-            /** tuple type for function arguments **/
-            using args_tuple = std::tuple<Args...>;
-            /** number of arguments **/
-            static constexpr std::size_t n_args = sizeof...(Args);
-
-            /** arg_type<i> is the type of the i'th argument to Fn **/
-            template <std::size_t I>
-            using arg_type = std::tuple_element_t<I, args_tuple>;
-        };
-
-        /** specialization for function references **/
-        template <typename R, typename... Args>
-        struct FnTraits<R(&)(Args...)> : FnTraits<R(*)(Args...)> {};
-
-        /** specialization for plain function types **/
-        template <typename R, typename... Args>
-        struct FnTraits<R(Args...)> : FnTraits<R(*)(Args...)> {};
-
         template <typename Fn>
         class Primitive {
         public:
             using AAllocator = xo::mm::AAllocator;
             using AGCObject = xo::mm::AGCObject;
             using DArray = xo::scm::DArray;
-            using Traits = FnTraits<Fn>;
+            using Traits = detail::PmFnTraits<Fn>;
 
         public:
             Primitive(std::string_view name, Fn fn) : name_{name}, fn_{fn} {}
@@ -63,14 +69,15 @@ namespace xo {
             bool is_nary() const noexcept { return false; }
             static constexpr std::int32_t n_args() noexcept { return Traits::n_args; }
 
-            obj<AGCObject> apply_nocheck(const DArray * args) {
-                return _apply_nocheck(args,
+            obj<AGCObject> apply_nocheck(obj<ARuntimeContext> rcx, const DArray * args) {
+                return _apply_nocheck(rcx, args,
                                       std::make_index_sequence<Traits::n_args>{});
             }
 
         private:
             template <std::size_t... Is>
-            obj<AGCObject> _apply_nocheck(const DArray * args,
+            obj<AGCObject> _apply_nocheck(obj<ARuntimeContext> rcx,
+                                          const DArray * args,
                                           std::index_sequence<Is...>)
             {
                 using xo::facet::FacetRegistry;
@@ -80,12 +87,11 @@ namespace xo {
                 assert(args);
                 assert(args->size() > 0);
 
-                obj<ARuntimeContext> rcx
-                    = FacetRegistry::instance().variant<ARuntimeContext>(args->at(0));
                 obj<AAllocator> mm = rcx.allocator();
 
                 R result
-                    = fn_(GCObjectConversion<typename Traits::template arg_type<Is>>::from_gco(mm, args->at(Is))... );
+                    = fn_(rcx,
+                          GCObjectConversion<typename Traits::template arg_type<Is>>::from_gco(mm, args->at(Is))... );
 
                 return GCObjectConversion<R>::to_gco(mm, result);
             }
diff --git a/xo-procedure2/include/xo/procedure2/DPrimitive_gco_2_gco_gco.hpp b/xo-procedure2/include/xo/procedure2/DPrimitive_gco_2_gco_gco.hpp
index 24ad3b41..78dd26b9 100644
--- a/xo-procedure2/include/xo/procedure2/DPrimitive_gco_2_gco_gco.hpp
+++ b/xo-procedure2/include/xo/procedure2/DPrimitive_gco_2_gco_gco.hpp
@@ -1,3 +1,8 @@
+/** @file DPrimitive_gco_2_gco_gco.hpp
+ *
+ *  @author Roland Conybeare, Jan 2026
+ **/
+
 #pragma once
 
 #include <xo/gc/GCObject.hpp>
@@ -13,3 +18,5 @@ namespace xo {
                                                                       obj<AGCObject>)>;
     }
 }
+
+/* end DPrimitive_gco_2_gco_gco.hpp */
diff --git a/xo-procedure2/include/xo/procedure2/Procedure.hpp b/xo-procedure2/include/xo/procedure2/Procedure.hpp
index 8b2cd91a..784608ed 100644
--- a/xo-procedure2/include/xo/procedure2/Procedure.hpp
+++ b/xo-procedure2/include/xo/procedure2/Procedure.hpp
@@ -2,7 +2,7 @@
  *
  *  Generated automagically from ingredients:
  *  1. code generator:
- *       [/Users/roland/proj/xo-umbrella2/xo-facet/codegen/genfacet]
+ *       [/home/roland/proj/xo-umbrella2-claude1/xo-facet/codegen/genfacet]
  *     arguments:
  *       --input [idl/Procedure.json5]
  *  2. jinja2 template for facet .hpp file:
diff --git a/xo-procedure2/include/xo/procedure2/RuntimeContext.hpp b/xo-procedure2/include/xo/procedure2/RuntimeContext.hpp
index 2e8300ed..3241451b 100644
--- a/xo-procedure2/include/xo/procedure2/RuntimeContext.hpp
+++ b/xo-procedure2/include/xo/procedure2/RuntimeContext.hpp
@@ -2,7 +2,7 @@
  *
  *  Generated automagically from ingredients:
  *  1. code generator:
- *       [/Users/roland/proj/xo-umbrella2/xo-facet/codegen/genfacet]
+ *       [/home/roland/proj/xo-umbrella2-claude1/xo-facet/codegen/genfacet]
  *     arguments:
  *       --input [idl/RuntimeContext.json5]
  *  2. jinja2 template for facet .hpp file:
diff --git a/xo-procedure2/include/xo/procedure2/detail/AProcedure.hpp b/xo-procedure2/include/xo/procedure2/detail/AProcedure.hpp
index 1be5e0d1..63a4da9b 100644
--- a/xo-procedure2/include/xo/procedure2/detail/AProcedure.hpp
+++ b/xo-procedure2/include/xo/procedure2/detail/AProcedure.hpp
@@ -2,7 +2,7 @@
  *
  *  Generated automagically from ingredients:
  *  1. code generator:
- *       [/Users/roland/proj/xo-umbrella2/xo-facet/codegen/genfacet]
+ *       [/home/roland/proj/xo-umbrella2-claude1/xo-facet/codegen/genfacet]
  *     arguments:
  *       --input [idl/Procedure.json5]
  *  2. jinja2 template for abstract facet .hpp file:
@@ -56,7 +56,7 @@ public:
 
     // nonconst methods
     /** invoke procedure; assume arguments satisfy type system **/
-    virtual obj<AGCObject> apply_nocheck(Opaque data, const DArray * args)  = 0;
+    virtual obj<AGCObject> apply_nocheck(Opaque data, obj<ARuntimeContext> rcx, const DArray * args)  = 0;
     ///@}
 }; /*AProcedure*/
 
diff --git a/xo-procedure2/include/xo/procedure2/detail/ARuntimeContext.hpp b/xo-procedure2/include/xo/procedure2/detail/ARuntimeContext.hpp
index 42030d9d..4e96bc8f 100644
--- a/xo-procedure2/include/xo/procedure2/detail/ARuntimeContext.hpp
+++ b/xo-procedure2/include/xo/procedure2/detail/ARuntimeContext.hpp
@@ -2,7 +2,7 @@
  *
  *  Generated automagically from ingredients:
  *  1. code generator:
- *       [/Users/roland/proj/xo-umbrella2/xo-facet/codegen/genfacet]
+ *       [/home/roland/proj/xo-umbrella2-claude1/xo-facet/codegen/genfacet]
  *     arguments:
  *       --input [idl/RuntimeContext.json5]
  *  2. jinja2 template for abstract facet .hpp file:
diff --git a/xo-procedure2/include/xo/procedure2/detail/IProcedure_Any.hpp b/xo-procedure2/include/xo/procedure2/detail/IProcedure_Any.hpp
index 3f66fcd5..5d711d4c 100644
--- a/xo-procedure2/include/xo/procedure2/detail/IProcedure_Any.hpp
+++ b/xo-procedure2/include/xo/procedure2/detail/IProcedure_Any.hpp
@@ -2,7 +2,7 @@
  *
  *  Generated automagically from ingredients:
  *  1. code generator:
- *       [/Users/roland/proj/xo-umbrella2/xo-facet/codegen/genfacet]
+ *       [/home/roland/proj/xo-umbrella2-claude1/xo-facet/codegen/genfacet]
  *     arguments:
  *       --input [idl/Procedure.json5]
  *  2. jinja2 template for abstract facet .hpp file:
@@ -60,7 +60,7 @@ namespace scm {
         [[noreturn]] std::int32_t n_args(Copaque)  const  noexcept override { _fatal(); }
 
         // nonconst methods
-        [[noreturn]] obj<AGCObject> apply_nocheck(Opaque, const DArray *)  override;
+        [[noreturn]] obj<AGCObject> apply_nocheck(Opaque, obj<ARuntimeContext>, const DArray *)  override;
 
         ///@}
 
diff --git a/xo-procedure2/include/xo/procedure2/detail/IProcedure_DPrimitive_gco_2_gco_gco.hpp b/xo-procedure2/include/xo/procedure2/detail/IProcedure_DPrimitive_gco_2_gco_gco.hpp
index 4b4351dd..11f2cd60 100644
--- a/xo-procedure2/include/xo/procedure2/detail/IProcedure_DPrimitive_gco_2_gco_gco.hpp
+++ b/xo-procedure2/include/xo/procedure2/detail/IProcedure_DPrimitive_gco_2_gco_gco.hpp
@@ -2,7 +2,7 @@
  *
  *  Generated automagically from ingredients:
  *  1. code generator:
- *       [/Users/roland/proj/xo-umbrella2/xo-facet/codegen/genfacet]
+ *       [/home/roland/proj/xo-umbrella2-claude1/xo-facet/codegen/genfacet]
  *     arguments:
  *       --input [idl/IProcedure_DPrimitive_gco_2_gco_gco.json5]
  *  2. jinja2 template for abstract facet .hpp file:
@@ -57,7 +57,7 @@ namespace xo {
 
             // non-const methods
             /** invoke procedure; assume arguments satisfy type system **/
-            static obj<AGCObject> apply_nocheck(DPrimitive_gco_2_gco_gco & self, const DArray * args);
+            static obj<AGCObject> apply_nocheck(DPrimitive_gco_2_gco_gco & self, obj<ARuntimeContext> rcx, const DArray * args);
             ///@}
         };
 
diff --git a/xo-procedure2/include/xo/procedure2/detail/IProcedure_Xfer.hpp b/xo-procedure2/include/xo/procedure2/detail/IProcedure_Xfer.hpp
index adf4995d..4f84652e 100644
--- a/xo-procedure2/include/xo/procedure2/detail/IProcedure_Xfer.hpp
+++ b/xo-procedure2/include/xo/procedure2/detail/IProcedure_Xfer.hpp
@@ -2,7 +2,7 @@
  *
  *  Generated automagically from ingredients:
  *  1. code generator:
- *       [/Users/roland/proj/xo-umbrella2/xo-facet/codegen/genfacet]
+ *       [/home/roland/proj/xo-umbrella2-claude1/xo-facet/codegen/genfacet]
  *     arguments:
  *       --input [idl/Procedure.json5]
  *  2. jinja2 template for abstract facet .hpp file:
@@ -50,8 +50,8 @@ namespace scm {
         }
 
         // non-const methods
-        obj<AGCObject> apply_nocheck(Opaque data, const DArray * args)  override {
-            return I::apply_nocheck(_dcast(data), args);
+        obj<AGCObject> apply_nocheck(Opaque data, obj<ARuntimeContext> rcx, const DArray * args)  override {
+            return I::apply_nocheck(_dcast(data), rcx, args);
         }
 
         ///@}
diff --git a/xo-procedure2/include/xo/procedure2/detail/IRuntimeContext_Any.hpp b/xo-procedure2/include/xo/procedure2/detail/IRuntimeContext_Any.hpp
index 419d45cc..1e5f97c0 100644
--- a/xo-procedure2/include/xo/procedure2/detail/IRuntimeContext_Any.hpp
+++ b/xo-procedure2/include/xo/procedure2/detail/IRuntimeContext_Any.hpp
@@ -2,7 +2,7 @@
  *
  *  Generated automagically from ingredients:
  *  1. code generator:
- *       [/Users/roland/proj/xo-umbrella2/xo-facet/codegen/genfacet]
+ *       [/home/roland/proj/xo-umbrella2-claude1/xo-facet/codegen/genfacet]
  *     arguments:
  *       --input [idl/RuntimeContext.json5]
  *  2. jinja2 template for abstract facet .hpp file:
diff --git a/xo-procedure2/include/xo/procedure2/detail/IRuntimeContext_Xfer.hpp b/xo-procedure2/include/xo/procedure2/detail/IRuntimeContext_Xfer.hpp
index e9f5512e..cd71b73e 100644
--- a/xo-procedure2/include/xo/procedure2/detail/IRuntimeContext_Xfer.hpp
+++ b/xo-procedure2/include/xo/procedure2/detail/IRuntimeContext_Xfer.hpp
@@ -2,7 +2,7 @@
  *
  *  Generated automagically from ingredients:
  *  1. code generator:
- *       [/Users/roland/proj/xo-umbrella2/xo-facet/codegen/genfacet]
+ *       [/home/roland/proj/xo-umbrella2-claude1/xo-facet/codegen/genfacet]
  *     arguments:
  *       --input [idl/RuntimeContext.json5]
  *  2. jinja2 template for abstract facet .hpp file:
diff --git a/xo-procedure2/include/xo/procedure2/detail/RProcedure.hpp b/xo-procedure2/include/xo/procedure2/detail/RProcedure.hpp
index 9fff0c6c..93599ae3 100644
--- a/xo-procedure2/include/xo/procedure2/detail/RProcedure.hpp
+++ b/xo-procedure2/include/xo/procedure2/detail/RProcedure.hpp
@@ -2,7 +2,7 @@
  *
  *  Generated automagically from ingredients:
  *  1. code generator:
- *       [/Users/roland/proj/xo-umbrella2/xo-facet/codegen/genfacet]
+ *       [/home/roland/proj/xo-umbrella2-claude1/xo-facet/codegen/genfacet]
  *     arguments:
  *       --input [idl/Procedure.json5]
  *  2. jinja2 template for abstract facet .hpp file:
@@ -56,8 +56,8 @@ public:
     }
 
     // non-const methods (still const in router!)
-    obj<AGCObject> apply_nocheck(const DArray * args)  {
-        return O::iface()->apply_nocheck(O::data(), args);
+    obj<AGCObject> apply_nocheck(obj<ARuntimeContext> rcx, const DArray * args)  {
+        return O::iface()->apply_nocheck(O::data(), rcx, args);
     }
 
     ///@}
diff --git a/xo-procedure2/include/xo/procedure2/detail/RRuntimeContext.hpp b/xo-procedure2/include/xo/procedure2/detail/RRuntimeContext.hpp
index 06b093de..b06227c8 100644
--- a/xo-procedure2/include/xo/procedure2/detail/RRuntimeContext.hpp
+++ b/xo-procedure2/include/xo/procedure2/detail/RRuntimeContext.hpp
@@ -2,7 +2,7 @@
  *
  *  Generated automagically from ingredients:
  *  1. code generator:
- *       [/Users/roland/proj/xo-umbrella2/xo-facet/codegen/genfacet]
+ *       [/home/roland/proj/xo-umbrella2-claude1/xo-facet/codegen/genfacet]
  *     arguments:
  *       --input [idl/RuntimeContext.json5]
  *  2. jinja2 template for abstract facet .hpp file:
diff --git a/xo-procedure2/src/procedure2/IProcedure_Any.cpp b/xo-procedure2/src/procedure2/IProcedure_Any.cpp
index 79b3b778..d28d98ee 100644
--- a/xo-procedure2/src/procedure2/IProcedure_Any.cpp
+++ b/xo-procedure2/src/procedure2/IProcedure_Any.cpp
@@ -35,7 +35,7 @@ IProcedure_Any::_valid
 // nonconst methods
 
 auto
-IProcedure_Any::apply_nocheck(Opaque, const DArray *)  -> obj<AGCObject>
+IProcedure_Any::apply_nocheck(Opaque, obj<ARuntimeContext>, const DArray *)  -> obj<AGCObject>
 {
     _fatal();
 }
diff --git a/xo-procedure2/src/procedure2/IProcedure_DPrimitive_gco_2_gco_gco.cpp b/xo-procedure2/src/procedure2/IProcedure_DPrimitive_gco_2_gco_gco.cpp
index 5266477c..67e73c97 100644
--- a/xo-procedure2/src/procedure2/IProcedure_DPrimitive_gco_2_gco_gco.cpp
+++ b/xo-procedure2/src/procedure2/IProcedure_DPrimitive_gco_2_gco_gco.cpp
@@ -2,7 +2,7 @@
  *
  *  Generated automagically from ingredients:
  *  1. code generator:
- *       [/Users/roland/proj/xo-umbrella2/xo-facet/codegen/genfacet]
+ *       [/home/roland/proj/xo-umbrella2-claude1/xo-facet/codegen/genfacet]
  *     arguments:
  *       --input [idl/IProcedure_DPrimitive_gco_2_gco_gco.json5]
  *  2. jinja2 template for abstract facet .hpp file:
@@ -28,9 +28,9 @@ namespace xo {
         }
 
         auto
-        IProcedure_DPrimitive_gco_2_gco_gco::apply_nocheck(DPrimitive_gco_2_gco_gco & self, const DArray * args) -> obj<AGCObject>
+        IProcedure_DPrimitive_gco_2_gco_gco::apply_nocheck(DPrimitive_gco_2_gco_gco & self, obj<ARuntimeContext> rcx, const DArray * args) -> obj<AGCObject>
         {
-            return self.apply_nocheck(args);
+            return self.apply_nocheck(rcx, args);
         }
 
     } /*namespace scm*/
diff --git a/xo-reader2/include/xo/reader2/DProgressSsm.hpp b/xo-reader2/include/xo/reader2/DProgressSsm.hpp
index 7258731c..31b3a55d 100644
--- a/xo-reader2/include/xo/reader2/DProgressSsm.hpp
+++ b/xo-reader2/include/xo/reader2/DProgressSsm.hpp
@@ -100,14 +100,12 @@ namespace xo {
                                        optype op);
 
             static void start(DArena & parser_mm,
-                              obj<AExpression> valex,
+                              obj<AExpression> lhs,
                               ParserStateMachine * p_psm);
-
-#ifdef NOT_YET
-            static void start(rp<Expression> valex,
+            static void start(DArena & parsermm,
+                              obj<AExpression> lhs,
                               optype optype,
-                              parserstatemachine * p_psm);
-#endif
+                              ParserStateMachine * p_psm);
 
             syntaxstatetype ssm_type() const noexcept;
 
@@ -147,6 +145,8 @@ namespace xo {
                                 ParserStateMachine * p_psm);
             void on_singleassign_token(const Token & tk,
                                        ParserStateMachine * p_psm);
+            void on_operator_token(const Token & tk,
+                                   ParserStateMachine * p_psm);
             void on_string_token(const Token & tk,
                                  ParserStateMachine * p_psm);
             void on_f64_token(const Token & tk,
@@ -207,17 +207,6 @@ namespace xo {
 
 
             void print(std::ostream & os) const override;
-
-        private:
-            /** assemble expression representing
-             *  value of
-             *  @code
-             *    f(lhs_, rhs_)
-             *  @endcode
-             *
-             *  where f determined by @ref op_type_
-             **/
-            obj<AExpression> assemble_expr(ParserStateMachine * p_psm);
 #endif
 
         private:
@@ -227,7 +216,14 @@ namespace xo {
             /** infix operator,  if supplied **/
             optype op_type_ = optype::invalid;
 
-            /** populate an expression here, following infix operator */
+            /** populate an expression here, that follows an infix operator.
+             *
+             *  Note this may not resolve immediately.
+             *  Consider input
+             *    5 + 6
+             *  Need to know if following token is *
+             *  before deciding if 6 belongs to addition 5 + 6
+             **/
             obj<AExpression> rhs_;
         };
     } /*namespace scm*/
diff --git a/xo-reader2/src/reader2/DExpectExprSsm.cpp b/xo-reader2/src/reader2/DExpectExprSsm.cpp
index fc5dbe08..49e95296 100644
--- a/xo-reader2/src/reader2/DExpectExprSsm.cpp
+++ b/xo-reader2/src/reader2/DExpectExprSsm.cpp
@@ -180,12 +180,11 @@ namespace xo {
             case tokentype::tk_in:
             case tokentype::tk_end:
             case tokentype::N:
+                p_psm->illegal_input_on_token("DExpectExprSsm::on_token",
+                                              tk,
+                                              this->get_expect_str());
                 break;
             }
-
-            p_psm->illegal_input_on_token("DExpectExprSsm::on_token",
-                                          tk,
-                                          this->get_expect_str());
         }
 
         void
diff --git a/xo-reader2/src/reader2/DProgressSsm.cpp b/xo-reader2/src/reader2/DProgressSsm.cpp
index 85f7b1d3..830dd30e 100644
--- a/xo-reader2/src/reader2/DProgressSsm.cpp
+++ b/xo-reader2/src/reader2/DProgressSsm.cpp
@@ -5,6 +5,15 @@
 
 #include "DProgressSsm.hpp"
 #include "ssm/ISyntaxStateMachine_DProgressSsm.hpp"
+
+#include "DExpectExprSsm.hpp"
+#include "ssm/ISyntaxStateMachine_DExpectExprSsm.hpp"
+
+#ifdef NOT_YET
+#include "DApplySsm.hpp"
+#include "ssm/ISyntaxStateMachine_DApplySsm.hpp"
+#endif
+
 #include <xo/printable2/Printable.hpp>
 #include <xo/facet/FacetRegistry.hpp>
 #include <xo/reflectutil/typeseq.hpp>
@@ -67,6 +76,7 @@ namespace xo {
             return "???";
         }
 
+        /** higher-precedence operators bind before lower-preference operators **/
         int
         precedence(optype x) {
             switch (x) {
@@ -97,6 +107,40 @@ namespace xo {
             return 0;
         }
 
+        namespace {
+            optype
+            tk2op(const tokentype & tktype) {
+                switch (tktype) {
+                case tokentype::tk_assign:
+                    return optype::op_assign;
+                case tokentype::tk_plus:
+                    return optype::op_add;
+                case tokentype::tk_minus:
+                    return optype::op_subtract;
+                case tokentype::tk_star:
+                    return optype::op_multiply;
+                case tokentype::tk_slash:
+                    return optype::op_divide;
+                case tokentype::tk_cmpeq:
+                    return optype::op_equal;
+                case tokentype::tk_cmpne:
+                    return optype::op_not_equal;
+                case tokentype::tk_leftangle:
+                    return optype::op_less;
+                case tokentype::tk_lessequal:
+                    return optype::op_less_equal;
+                case tokentype::tk_rightangle:
+                    return optype::op_great;
+                case tokentype::tk_greatequal:
+                    return optype::op_great_equal;
+                default:
+                    assert(false);
+                    return optype::invalid;
+                }
+                return optype::invalid;
+            }
+        }
+
         DProgressSsm *
         DProgressSsm::make(DArena & mm,
                            obj<AExpression> lhs,
@@ -112,26 +156,26 @@ namespace xo {
 
         void
         DProgressSsm::start(DArena & parser_mm,
-                            obj<AExpression> valex,
+                            obj<AExpression> lhs,
+                            optype op,
                             ParserStateMachine * p_psm)
         {
             DProgressSsm * progress_ssm
-                = DProgressSsm::make(parser_mm, valex, optype::invalid);
+                = DProgressSsm::make(parser_mm, lhs, op);
 
-            obj<ASyntaxStateMachine> ssm
-                = with_facet<ASyntaxStateMachine>::mkobj(progress_ssm);
+            obj<ASyntaxStateMachine,DProgressSsm> ssm(progress_ssm);
 
             p_psm->push_ssm(ssm);
         }
 
-#ifdef NOT_YET
         void
-        progress_xs::start(rp<Expression> valex, optype op, parserstatemachine * p_psm) {
-            p_psm->push_exprstate(progress_xs::make(valex, op));
+        DProgressSsm::start(DArena & parser_mm,
+                            obj<AExpression> lhs,
+                            ParserStateMachine * p_psm)
+        {
+            start(parser_mm, lhs, optype::invalid, p_psm);
         }
 
-#endif
-
         DProgressSsm::DProgressSsm(obj<AExpression> valex,
                                    optype op)
           : lhs_{valex},
@@ -225,12 +269,11 @@ namespace xo {
             case tokentype::tk_plus:
             case tokentype::tk_minus:
                 break;
+
             case tokentype::tk_star:
-#ifdef NOT_YET
                 this->on_operator_token(tk, p_psm);
                 return;
-#endif
-                break;
+
             case tokentype::tk_slash:
             case tokentype::tk_cmpeq:
             case tokentype::tk_cmpne:
@@ -295,6 +338,75 @@ namespace xo {
                                           this->get_expect_str());
         }
 
+        void
+        DProgressSsm::on_operator_token(const Token & tk,
+                                        ParserStateMachine * p_psm)
+        {
+            if (op_type_ == optype::invalid) {
+                // tk is the operator this instance was waiting for
+                this->op_type_ = tk2op(tk.tk_type());
+
+                DExpectExprSsm::start(p_psm->parser_alloc(), p_psm);
+                return;
+            } else if (rhs_) {
+                optype op_type2 = tk2op(tk.tk_type());
+
+                /* already have {lhs_, op_type_, rhs_},
+                 * with incoming op_type2 operator decides whether to parse like
+                 *    (lhs_, op_type_, rhs_) op_type2 ...
+                 * or
+                 *    (lhs_, op_type_, (rhs_ op_type2 ...))
+                 */
+
+                if (precedence(op_type_) >= precedence(op_type2)) {
+                    /* associate to the left
+                     *
+                     * parse              like
+                     *   a + b - ...        (a + b) - ...
+                     *   a * b - ...        (a * b) - ...
+                     */
+
+                    auto lhs2 = this->assemble_expr(p_psm);
+
+                    p_psm->pop_ssm();
+
+                    DProgressSsm::start(p_psm->parser_alloc(),
+                                        lhs2,
+                                        op_type2,
+                                        p_psm);
+                    return;
+                } else {
+                    /* associate to the right
+                     *
+                     * parse             like
+                     *  a + b * ...        (a + (b * ...))
+                     */
+
+                    p_psm->pop_ssm();
+
+                    /* (a + ..) */
+                    DProgressSsm::start(p_psm->parser_alloc(),
+                                        lhs_,
+                                        op_type_,
+                                        p_psm);
+                    DExpectExprSsm::start(p_psm->parser_alloc(),
+                                          p_psm);
+                    /* (b * ..) */
+                    DProgressSsm::start(p_psm->parser_alloc(),
+                                        rhs_,
+                                        op_type2,
+                                        p_psm);
+                    DExpectExprSsm::start(p_psm->parser_alloc(),
+                                          p_psm);
+                    return;
+                }
+            }
+
+            p_psm->illegal_input_on_token("DProgressSsm::on_operator_token",
+                                          tk,
+                                          this->get_expect_str());
+        }
+
         void
         DProgressSsm::on_string_token(const Token & tk,
                                       ParserStateMachine * p_psm)
@@ -905,112 +1017,10 @@ namespace xo {
              *   { n * n }
              */
         }
+#endif
 
-        namespace {
-            optype
-            tk2op(const tokentype & tktype) {
-                switch (tktype) {
-                case tokentype::tk_assign:
-                    return optype::op_assign;
-                case tokentype::tk_plus:
-                    return optype::op_add;
-                case tokentype::tk_minus:
-                    return optype::op_subtract;
-                case tokentype::tk_star:
-                    return optype::op_multiply;
-                case tokentype::tk_slash:
-                    return optype::op_divide;
-                case tokentype::tk_cmpeq:
-                    return optype::op_equal;
-                case tokentype::tk_cmpne:
-                    return optype::op_not_equal;
-                case tokentype::tk_leftangle:
-                    return optype::op_less;
-                case tokentype::tk_lessequal:
-                    return optype::op_less_equal;
-                case tokentype::tk_rightangle:
-                    return optype::op_great;
-                case tokentype::tk_greatequal:
-                    return optype::op_great_equal;
-                default:
-                    assert(false);
-                    return optype::invalid;
-                }
-                return optype::invalid;
-            }
-        }
-
-        void
-        progress_xs::on_operator_token(const token_type & tk,
-                                       parserstatemachine * p_psm)
-        {
-            scope log(XO_DEBUG(p_psm->debug_flag()));
-
-            constexpr const char * c_self_name = "progress_xs::on_operator_token";
-
-            if (op_type_ == optype::invalid) {
-                this->op_type_ = tk2op(tk.tk_type());
-
-                /* infix operator must be followed by non-empty expression */
-                expect_expr_xs::start(p_psm);
-            } else if (rhs_) {
-                /* already have complete expression stashed.
-                 * behavior depends on operator precedence for tk with stored operator
-                 * this->op_type_
-                 */
-                optype op2 = tk2op(tk.tk_type());
-
-                if (precedence(op2) <= precedence(this->op_type_)) {
-                    /* e.g.
-                     *   6.2 * 4.9 + ...
-                     *
-                     * in stack:
-                     *   1. progress_xs lhs=6.2, op=*, rhs=4.9
-                     *
-                     * out stack
-                     *   1. progress_xs lhs=apply(*,6.2,4.9), op=+
-                     */
-
-                    /* 1. instantiate expression for *this */
-                    auto expr = this->assemble_expr(p_psm);
-
-                    /* 2. remove from stack */
-                    std::unique_ptr<exprstate> self  = p_psm->pop_exprstate();
-
-                    /* 3. replace with new progress_xs: */
-                    progress_xs::start(expr, op2, p_psm);
-
-                    /* infix operator must be followed by non-empty expression */
-                    expect_expr_xs::start(p_psm);
-                } else {
-                    /* e.g.
-                     *   6.2 + 4.9 * ...
-                     *
-                     * in stack:
-                     *   1. progress_xs lhs=6.2, op=+, rhs=4.9
-                     *
-                     * out stack:
-                     *   1. progress_xs lhs=6.2, op=+
-                     *   2. expect_rhs_expression
-                     *   3. progress_xs lhs=4.9, op=*
-                     *   4. expect_rhs_expression
-                     */
-
-                    std::unique_ptr<exprstate> self = p_psm->pop_exprstate();
-
-                    /* 1. replace with nested incomplete infix exprs */
-                    progress_xs::start(lhs_, op_type_, p_psm);
-                    expect_expr_xs::start(p_psm);
-                    progress_xs::start(rhs_, op2, p_psm);
-                    expect_expr_xs::start(p_psm);
-                }
-
-            } else {
-                throw std::runtime_error(tostr(c_self_name,
-                                               ": expected expression following operator",
-                                               xtag("tk", tk)));
-            }
-        }
+#ifdef OBSOLETE
+        //void progress_xs::on_operator_token(const token_type & tk, parserstatemachine * p_psm)
 
         void
         progress_xs::on_bool_token(const token_type & tk,
@@ -1163,7 +1173,25 @@ namespace xo {
             case optype::op_great_equal:
             case optype::op_add:
             case optype::op_subtract:
+                break;
+
             case optype::op_multiply:
+#ifdef NOT_YET
+                {
+                    /* note:
+                     * 1. don't assume we know lhs_ / rhs_ value types yet.
+                     *    perhaps have expression like
+                     *      f(..) * g(..)
+                     *    where f is the function that contains current ssm.
+                     * 2. consequence: we need representation for
+                     *    polymorphic multiply on unknown numeric arguments.
+                     */
+
+                    DApplyExpr::make2();
+                }
+#endif
+
+                break;
             case optype::op_divide:
                 // TODO: implement binary operator expression assembly
                 break;