diff --git a/xo-reader/include/xo/reader/define_xs.hpp b/xo-reader/include/xo/reader/define_xs.hpp
index 4f2eb66a..0c6d46c8 100644
--- a/xo-reader/include/xo/reader/define_xs.hpp
+++ b/xo-reader/include/xo/reader/define_xs.hpp
@@ -79,9 +79,9 @@ namespace xo {
 
             defexprstatetype defxs_type() const { return defxs_type_; }
 
-            virtual void on_expr(ref::brw<Expression> expr,
+            virtual void on_expr(bp<Expression> expr,
                                  parserstatemachine * p_psm) override;
-            virtual void on_expr_with_semicolon(ref::brw<Expression> expr,
+            virtual void on_expr_with_semicolon(bp<Expression> expr,
                                                 parserstatemachine * p_psm) override;
             virtual void on_symbol(const std::string & symbol_name,
                                    parserstatemachine * p_psm) override;
diff --git a/xo-reader/include/xo/reader/envframe.hpp b/xo-reader/include/xo/reader/envframe.hpp
index a0fcfb7d..a6bfee29 100644
--- a/xo-reader/include/xo/reader/envframe.hpp
+++ b/xo-reader/include/xo/reader/envframe.hpp
@@ -25,11 +25,14 @@ namespace xo {
 
             const std::vector<rp<Variable>> & argl() const { return argl_; }
 
-            /** lookup variable by name.  If found, return it.
+            /** lookup variable by @p name.  If found, return it.
              *  Otherwise return nullptr
              **/
             rp<Variable> lookup(const std::string & name) const;
 
+            /** establish (replacing if already exists) binding for variable @p var **/
+            void upsert(bp<Variable> var);
+
             void print (std::ostream & os) const;
 
         private:
@@ -44,5 +47,4 @@ namespace xo {
     } /*namespace scm*/
 } /*namespace xo*/
 
-
 /* end envframe.hpp */
diff --git a/xo-reader/include/xo/reader/envframestack.hpp b/xo-reader/include/xo/reader/envframestack.hpp
index e438f47f..60314756 100644
--- a/xo-reader/include/xo/reader/envframestack.hpp
+++ b/xo-reader/include/xo/reader/envframestack.hpp
@@ -28,6 +28,11 @@ namespace xo {
              **/
             rp<Variable> lookup(const std::string & x) const;
 
+            /** update/replace binding for variable @p target.
+             *  New binding may have a different type.
+             **/
+            void upsert(bp<Variable> target);
+
             envframe & top_envframe();
             void push_envframe(envframe x);
             void pop_envframe();
diff --git a/xo-reader/include/xo/reader/expect_expr_xs.hpp b/xo-reader/include/xo/reader/expect_expr_xs.hpp
index 900ee1de..b416c7eb 100644
--- a/xo-reader/include/xo/reader/expect_expr_xs.hpp
+++ b/xo-reader/include/xo/reader/expect_expr_xs.hpp
@@ -48,12 +48,12 @@ namespace xo {
                                       parserstatemachine * p_psm) override;
 
             /** update exprstate in response to a successfully-parsed subexpression **/
-            virtual void on_expr(ref::brw<Expression> expr,
+            virtual void on_expr(bp<Expression> expr,
                                  parserstatemachine * p_psm) override;
             /** update exprstate in response to a successfully-parsed subexpression,
              *  that's terminated by semicolon ';'
              **/
-            virtual void on_expr_with_semicolon(ref::brw<Expression> expr,
+            virtual void on_expr_with_semicolon(bp<Expression> expr,
                                                 parserstatemachine * p_psm) override;
 
 
diff --git a/xo-reader/include/xo/reader/exprseq_xs.hpp b/xo-reader/include/xo/reader/exprseq_xs.hpp
index 52ec315c..c41e3cf3 100644
--- a/xo-reader/include/xo/reader/exprseq_xs.hpp
+++ b/xo-reader/include/xo/reader/exprseq_xs.hpp
@@ -56,9 +56,9 @@ namespace xo {
 
             virtual void on_typedescr(TypeDescr td,
                                       parserstatemachine * p_psm) override;
-            virtual void on_expr(ref::brw<Expression> expr,
+            virtual void on_expr(bp<Expression> expr,
                                  parserstatemachine * p_psm) override;
-            virtual void on_expr_with_semicolon(ref::brw<Expression> expr,
+            virtual void on_expr_with_semicolon(bp<Expression> expr,
                                                 parserstatemachine * p_psm) override;
 
         private:
diff --git a/xo-reader/include/xo/reader/exprstate.hpp b/xo-reader/include/xo/reader/exprstate.hpp
index 467348df..f6d592db 100644
--- a/xo-reader/include/xo/reader/exprstate.hpp
+++ b/xo-reader/include/xo/reader/exprstate.hpp
@@ -106,11 +106,11 @@ namespace xo {
                           parserstatemachine * p_psm);
 
             /** update exprstate in response to a successfully-parsed subexpression **/
-            virtual void on_expr(ref::brw<Expression> expr,
+            virtual void on_expr(bp<Expression> expr,
                                  parserstatemachine * p_psm);
 
             /** update exprstate in response to a successfully-parsed subexpression, that ends with semicolon **/
-            virtual void on_expr_with_semicolon(ref::brw<Expression> expr,
+            virtual void on_expr_with_semicolon(bp<Expression> expr,
                                                 parserstatemachine * p_psm);
 
             /** update exprstate when expecting a symbol **/
@@ -189,6 +189,11 @@ namespace xo {
             void illegal_input_error(const char * self_name,
                                      const token_type & tk) const;
 
+            /** throw exception when unable to locate definition for a variable
+             **/
+            void unknown_variable_error(const char * self_name,
+                                        const token_type & tk) const;
+
         protected:
             /** explicit subtype: identifies derived class **/
             exprstatetype exs_type_;
diff --git a/xo-reader/include/xo/reader/lambda_xs.hpp b/xo-reader/include/xo/reader/lambda_xs.hpp
index 728bc9e3..43f161a1 100644
--- a/xo-reader/include/xo/reader/lambda_xs.hpp
+++ b/xo-reader/include/xo/reader/lambda_xs.hpp
@@ -55,9 +55,9 @@ namespace xo {
                                          parserstatemachine * p_psm) override;
             virtual void on_formal_arglist(const std::vector<rp<Variable>> & argl,
                                            parserstatemachine * p_psm) override;
-            virtual void on_expr(ref::brw<Expression> expr,
+            virtual void on_expr(bp<Expression> expr,
                                  parserstatemachine * p_psm) override;
-            virtual void on_expr_with_semicolon(ref::brw<Expression> expr,
+            virtual void on_expr_with_semicolon(bp<Expression> expr,
                                                 parserstatemachine * p_psm) override;
             virtual void on_semicolon_token(const token_type & tk,
                                             parserstatemachine * p_psm) override;
diff --git a/xo-reader/include/xo/reader/let1_xs.hpp b/xo-reader/include/xo/reader/let1_xs.hpp
index 4486869f..b510f50e 100644
--- a/xo-reader/include/xo/reader/let1_xs.hpp
+++ b/xo-reader/include/xo/reader/let1_xs.hpp
@@ -25,7 +25,7 @@ namespace xo {
                               const rp<Expression> & rhs,
                               parserstatemachine * p_psm);
 
-            virtual void on_expr(ref::brw<Expression> expr,
+            virtual void on_expr(bp<Expression> expr,
                                  parserstatemachine * p_psm) override;
 
             virtual void on_rightbrace_token(const token_type & tk,
diff --git a/xo-reader/include/xo/reader/paren_xs.hpp b/xo-reader/include/xo/reader/paren_xs.hpp
index 0d251a76..ab7da4fc 100644
--- a/xo-reader/include/xo/reader/paren_xs.hpp
+++ b/xo-reader/include/xo/reader/paren_xs.hpp
@@ -43,7 +43,7 @@ namespace xo {
             bool admits_f64() const;
             bool admits_rightparen() const;
 
-            virtual void on_expr(ref::brw<Expression> expr,
+            virtual void on_expr(bp<Expression> expr,
                                  parserstatemachine * p_psm) override;
             virtual void on_symbol(const std::string & symbol,
                                    parserstatemachine * p_psm) override;
diff --git a/xo-reader/include/xo/reader/parser.hpp b/xo-reader/include/xo/reader/parser.hpp
index 01a3b9b9..850a5513 100644
--- a/xo-reader/include/xo/reader/parser.hpp
+++ b/xo-reader/include/xo/reader/parser.hpp
@@ -154,7 +154,7 @@ namespace xo {
             /** create parser in initial state;
              *  parser is ready to receive tokens via @ref include_token
              **/
-            parser() = default;
+            parser();
 
             /** for diagnostics: number of entries in parser stack **/
             std::size_t stack_size() const { return xs_stack_.size(); }
diff --git a/xo-reader/include/xo/reader/parserstatemachine.hpp b/xo-reader/include/xo/reader/parserstatemachine.hpp
index 0e69d44c..24868e12 100644
--- a/xo-reader/include/xo/reader/parserstatemachine.hpp
+++ b/xo-reader/include/xo/reader/parserstatemachine.hpp
@@ -40,13 +40,18 @@ namespace xo {
              **/
             rp<Variable> lookup_var(const std::string & x) const;
 
+            /** update/replace binding for variable @p x in lexical context
+             *  represented by this psm.  Always acts on innermost frame.
+             **/
+            void upsert_var(bp<Variable> x);
+
             void push_envframe(envframe x);
             void pop_envframe();
 
             // ----- parsing outputs -----
 
-            void on_expr(ref::brw<Expression> expr);
-            void on_expr_with_semicolon(ref::brw<Expression> expr);
+            void on_expr(bp<Expression> expr);
+            void on_expr_with_semicolon(bp<Expression> expr);
             void on_symbol(const std::string & symbol);
 
             // ---- parsing inputs -----
diff --git a/xo-reader/include/xo/reader/progress_xs.hpp b/xo-reader/include/xo/reader/progress_xs.hpp
index 03bd933e..4277ba73 100644
--- a/xo-reader/include/xo/reader/progress_xs.hpp
+++ b/xo-reader/include/xo/reader/progress_xs.hpp
@@ -47,6 +47,10 @@ namespace xo {
 
         /** @class progress_xs
          *  @brief state machine for parsing a schematica runtime-value-expression
+         *
+         *  Handles an expression that produces a value, for example appearing on the
+         *  right-hand side of a definition.
+         *
          **/
         class progress_xs : public exprstate {
         public:
@@ -65,7 +69,7 @@ namespace xo {
 
             bool admits_f64() const;
 
-            virtual void on_expr(ref::brw<Expression> expr,
+            virtual void on_expr(bp<Expression> expr,
                                  parserstatemachine * p_psm) override;
             virtual void on_symbol_token(const token_type & tk,
                                          parserstatemachine * p_psm) override;
diff --git a/xo-reader/include/xo/reader/sequence_xs.hpp b/xo-reader/include/xo/reader/sequence_xs.hpp
index b361bbf7..fd16ffb5 100644
--- a/xo-reader/include/xo/reader/sequence_xs.hpp
+++ b/xo-reader/include/xo/reader/sequence_xs.hpp
@@ -24,7 +24,7 @@ namespace xo {
              **/
             static void start(parserstatemachine * p_psm);
 
-            virtual void on_expr(ref::brw<Expression> expr,
+            virtual void on_expr(bp<Expression> expr,
                                  parserstatemachine * p_psm) override;
 
             virtual void on_rightbrace_token(const token_type & tk,
diff --git a/xo-reader/src/reader/define_xs.cpp b/xo-reader/src/reader/define_xs.cpp
index fdd3c410..a6faea78 100644
--- a/xo-reader/src/reader/define_xs.cpp
+++ b/xo-reader/src/reader/define_xs.cpp
@@ -58,7 +58,7 @@ namespace xo {
         {}
 
         void
-        define_xs::on_expr(ref::brw<Expression> expr,
+        define_xs::on_expr(bp<Expression> expr,
                            parserstatemachine * p_psm)
         {
             constexpr bool c_debug_flag = true;
@@ -90,7 +90,7 @@ namespace xo {
         }
 
         void
-        define_xs::on_expr_with_semicolon(ref::brw<Expression> expr,
+        define_xs::on_expr_with_semicolon(bp<Expression> expr,
                                           parserstatemachine * p_psm)
         {
             this->on_expr(expr, p_psm);
@@ -186,11 +186,16 @@ namespace xo {
             log && log("defxs_type", defxs_type_);
 
             if (this->defxs_type_ == defexprstatetype::def_6) {
-                rp<Expression> expr = this->def_expr_;
+                rp<DefineExprAccess> def_expr = this->def_expr_;
 
                 std::unique_ptr<exprstate> self = p_psm->pop_exprstate();
 
-                p_psm->top_exprstate().on_expr(expr, p_psm);
+                /* remember variable binding in lexical context,
+                 * so we can refer to it later
+                 */
+                p_psm->upsert_var(def_expr->lhs_variable());
+
+                p_psm->top_exprstate().on_expr(def_expr, p_psm);
             } else {
                 exprstate::on_semicolon_token(tk, p_psm);
             }
diff --git a/xo-reader/src/reader/envframe.cpp b/xo-reader/src/reader/envframe.cpp
index 38ff1fcb..c0e39693 100644
--- a/xo-reader/src/reader/envframe.cpp
+++ b/xo-reader/src/reader/envframe.cpp
@@ -20,6 +20,20 @@ namespace xo {
             return nullptr;
         }
 
+        void
+        envframe::upsert(bp<Variable> target) {
+            for (auto & var : this->argl_) {
+                if (var->name() == target->name()) {
+                    /* replace existing variable -- may change type */
+                    var = target.promote();
+                    return;
+                }
+            }
+
+            /* here: target not already present in this frame, append it */
+            this->argl_.push_back(target.promote());
+        }
+
         void
         envframe::print(std::ostream & os) const {
             os << "<envframe"
diff --git a/xo-reader/src/reader/envframestack.cpp b/xo-reader/src/reader/envframestack.cpp
index 047e1789..33afd306 100644
--- a/xo-reader/src/reader/envframestack.cpp
+++ b/xo-reader/src/reader/envframestack.cpp
@@ -66,6 +66,14 @@ namespace xo {
             return nullptr;
         }
 
+        void
+        envframestack::upsert(bp<Variable> x) {
+            /* upsert should always happen in the innermost lexical context.
+             * We are providing new variable binding (perhaps shadowing an existing binding)
+             */
+            this->top_envframe().upsert(x);
+        }
+
         void
         envframestack::print(std::ostream & os) const {
             os << "<envframestack"
diff --git a/xo-reader/src/reader/expect_expr_xs.cpp b/xo-reader/src/reader/expect_expr_xs.cpp
index c4a56f30..cf476baf 100644
--- a/xo-reader/src/reader/expect_expr_xs.cpp
+++ b/xo-reader/src/reader/expect_expr_xs.cpp
@@ -203,7 +203,7 @@ namespace xo {
         }
 
         void
-        expect_expr_xs::on_expr(ref::brw<Expression> expr,
+        expect_expr_xs::on_expr(bp<Expression> expr,
                                 parserstatemachine * p_psm)
         {
             constexpr bool c_debug_flag = true;
@@ -218,7 +218,7 @@ namespace xo {
         } /*on_expr*/
 
         void
-        expect_expr_xs::on_expr_with_semicolon(ref::brw<Expression> expr,
+        expect_expr_xs::on_expr_with_semicolon(bp<Expression> expr,
                                                parserstatemachine * p_psm)
         {
             constexpr bool c_debug_flag = true;
diff --git a/xo-reader/src/reader/exprseq_xs.cpp b/xo-reader/src/reader/exprseq_xs.cpp
index e650e238..bf5fea79 100644
--- a/xo-reader/src/reader/exprseq_xs.cpp
+++ b/xo-reader/src/reader/exprseq_xs.cpp
@@ -4,9 +4,11 @@
 #include "parserstatemachine.hpp"
 #include "exprstatestack.hpp"
 #include "exprseq_xs.hpp"
-#include "expect_expr_xs.hpp"
+//#include "expect_expr_xs.hpp"
+#include "progress_xs.hpp"
 #include "define_xs.hpp"
 #include "expect_symbol_xs.hpp"
+#include "xo/expression/Constant.hpp"
 
 namespace xo {
     namespace scm {
@@ -47,17 +49,41 @@ namespace xo {
 
         void
         exprseq_xs::on_symbol_token(const token_type & tk,
-                                    parserstatemachine * /*p_psm*/)
+                                    parserstatemachine * p_psm)
         {
             constexpr const char * c_self_name = "exprseq_xs::on_symbol_token";
 
-            this->illegal_input_error(c_self_name, tk);
+            if (xseqtype_ == exprseqtype::toplevel_interactive)
+            {
+                /* In interactive session, allow top-level variable reference.
+                 * This could be:
+                 *   a;      // variable references
+                 *   a = 1;  // single assignment
+                 *   a == 1; // rhs expression
+                 *   a + b;  // rhs expression
+                 * Variable must have been defined!
+                 */
+                rp<Variable> var = p_psm->lookup_var(tk.text());
+
+                if (var.get()) {
+                    progress_xs::start(var, p_psm);
+                } else {
+                    this->unknown_variable_error(c_self_name, tk);
+                }
+            } else {
+                /* policy: don't allow variable references as toplevel expressions
+                 * unless interactive session
+                 */
+                this->illegal_input_error(c_self_name, tk);
+            }
         }
 
         void
         exprseq_xs::on_i64_token(const token_type & tk,
                                  parserstatemachine * p_psm)
         {
+            using xo::ast::Constant;
+
             constexpr bool c_debug_flag = true;
             scope log(XO_DEBUG(c_debug_flag));
 
@@ -65,9 +91,11 @@ namespace xo {
 
             if (xseqtype_ == exprseqtype::toplevel_interactive)
             {
-                expect_expr_xs::start(p_psm);
-                p_psm->top_exprstate().on_i64_token(tk, p_psm);
+                progress_xs::start(Constant<int64_t>::make(tk.i64_value()), p_psm);
             } else {
+                /* policy: don't allow literals as toplevel expressions
+                 * unless interactive session.
+                 */
                 this->illegal_input_error(c_self_name, tk);
             }
         }
@@ -82,7 +110,7 @@ namespace xo {
         }
 
         void
-        exprseq_xs::on_expr(ref::brw<Expression> expr,
+        exprseq_xs::on_expr(bp<Expression> expr,
                             parserstatemachine * p_psm)
         {
             /* toplevel expression sequence accepts an
@@ -95,7 +123,7 @@ namespace xo {
         } /*on_expr*/
 
         void
-        exprseq_xs::on_expr_with_semicolon(ref::brw<Expression> expr,
+        exprseq_xs::on_expr_with_semicolon(bp<Expression> expr,
                                            parserstatemachine * p_psm)
         {
             /* toplevel expression sequence accepts an
diff --git a/xo-reader/src/reader/exprstate.cpp b/xo-reader/src/reader/exprstate.cpp
index 8145c550..c255a182 100644
--- a/xo-reader/src/reader/exprstate.cpp
+++ b/xo-reader/src/reader/exprstate.cpp
@@ -58,8 +58,13 @@ namespace xo {
 
         void
         exprstate::on_def_token(const token_type & tk,
-                                parserstatemachine * /*p_psm*/)
+                                parserstatemachine * p_psm)
         {
+            constexpr bool c_debug_flag = true;
+            scope log(XO_DEBUG(c_debug_flag));
+
+            log && log(xtag("exstype", p_psm->top_exprstate().exs_type()));
+
             this->illegal_input_error("exprstate::on_def_token", tk);
         }
 
@@ -387,7 +392,7 @@ namespace xo {
         }
 
         void
-        exprstate::on_expr(ref::brw<Expression> expr,
+        exprstate::on_expr(bp<Expression> expr,
                            parserstatemachine * /*p_psm*/)
         {
             constexpr bool c_debug_flag = true;
@@ -400,7 +405,7 @@ namespace xo {
         } /*on_expr*/
 
         void
-        exprstate::on_expr_with_semicolon(ref::brw<Expression> expr,
+        exprstate::on_expr_with_semicolon(bp<Expression> expr,
                                           parserstatemachine * /*p_psm*/)
         {
             constexpr bool c_debug_flag = true;
@@ -454,6 +459,16 @@ namespace xo {
                        xtag("token", tk),
                        xtag("state", *this)));
         }
+
+        void
+        exprstate::unknown_variable_error(const char * self_name,
+                                          const token_type & tk) const
+        {
+            throw std::runtime_error
+                (tostr(self_name,
+                       ": unknown variable name",
+                       xtag("var", tk.text())));
+        }
     } /*namespace scm*/
 } /*namespace xo*/
 
diff --git a/xo-reader/src/reader/lambda_xs.cpp b/xo-reader/src/reader/lambda_xs.cpp
index ab9b73d8..c7d664f7 100644
--- a/xo-reader/src/reader/lambda_xs.cpp
+++ b/xo-reader/src/reader/lambda_xs.cpp
@@ -71,7 +71,7 @@ namespace xo {
         }
 
         void
-        lambda_xs::on_expr(ref::brw<Expression> expr,
+        lambda_xs::on_expr(bp<Expression> expr,
                            parserstatemachine * p_psm)
         {
             if (lmxs_type_ == lambdastatetype::lm_2) {
@@ -83,7 +83,7 @@ namespace xo {
         }
 
         void
-        lambda_xs::on_expr_with_semicolon(ref::brw<Expression> expr,
+        lambda_xs::on_expr_with_semicolon(bp<Expression> expr,
                                           parserstatemachine * p_psm)
         {
             this->on_expr(expr, p_psm);
diff --git a/xo-reader/src/reader/let1_xs.cpp b/xo-reader/src/reader/let1_xs.cpp
index 0afbd415..f1f4e1b1 100644
--- a/xo-reader/src/reader/let1_xs.cpp
+++ b/xo-reader/src/reader/let1_xs.cpp
@@ -54,13 +54,13 @@ namespace xo {
         {}
 
         void
-        let1_xs::on_expr(ref::brw<Expression> expr,
+        let1_xs::on_expr(bp<Expression> expr,
                          parserstatemachine * p_psm)
         {
             constexpr bool c_debug_flag = true;
             scope log(XO_DEBUG(c_debug_flag));
 
-            ref::brw<DefineExpr> def_expr = DefineExpr::from(expr);
+            bp<DefineExpr> def_expr = DefineExpr::from(expr);
 
             if (def_expr) {
                 /** nested_start: control returns via
diff --git a/xo-reader/src/reader/paren_xs.cpp b/xo-reader/src/reader/paren_xs.cpp
index 357d377a..8ac163ea 100644
--- a/xo-reader/src/reader/paren_xs.cpp
+++ b/xo-reader/src/reader/paren_xs.cpp
@@ -195,7 +195,7 @@ namespace xo {
         }
 
         void
-        paren_xs::on_expr(ref::brw<Expression> expr,
+        paren_xs::on_expr(bp<Expression> expr,
                           parserstatemachine * p_psm)
         {
             constexpr bool c_debug_flag = true;
diff --git a/xo-reader/src/reader/parser.cpp b/xo-reader/src/reader/parser.cpp
index 4250e028..7495077c 100644
--- a/xo-reader/src/reader/parser.cpp
+++ b/xo-reader/src/reader/parser.cpp
@@ -24,6 +24,15 @@ namespace xo {
     namespace scm {
         // ----- parser -----
 
+        parser::parser()
+            : xs_stack_{}, env_stack_{}
+        {
+            /* top-level environment.  initially empty */
+            envframe toplevel_env;
+
+            this->env_stack_.push_envframe(toplevel_env);
+        }
+
         bool
         parser::has_incomplete_expr() const {
             return !xs_stack_.empty();
diff --git a/xo-reader/src/reader/parserstatemachine.cpp b/xo-reader/src/reader/parserstatemachine.cpp
index 373e83e3..c4be496f 100644
--- a/xo-reader/src/reader/parserstatemachine.cpp
+++ b/xo-reader/src/reader/parserstatemachine.cpp
@@ -15,6 +15,11 @@ namespace xo {
             return p_env_stack_->lookup(x);
         }
 
+        void
+        parserstatemachine::upsert_var(bp<Variable> x) {
+            p_env_stack_->upsert(x);
+        }
+
         std::unique_ptr<exprstate>
         parserstatemachine::pop_exprstate() {
             return p_stack_->pop_exprstate();
@@ -49,7 +54,7 @@ namespace xo {
         }
 
         void
-        parserstatemachine::on_expr(ref::brw<Expression> x)
+        parserstatemachine::on_expr(bp<Expression> x)
         {
             constexpr bool c_debug_flag = true;
             scope log(XO_DEBUG(c_debug_flag));
@@ -62,7 +67,7 @@ namespace xo {
         }
 
         void
-        parserstatemachine::on_expr_with_semicolon(ref::brw<Expression> x)
+        parserstatemachine::on_expr_with_semicolon(bp<Expression> x)
         {
             constexpr bool c_debug_flag = true;
             scope log(XO_DEBUG(c_debug_flag));
diff --git a/xo-reader/src/reader/progress_xs.cpp b/xo-reader/src/reader/progress_xs.cpp
index 44938fca..930ef734 100644
--- a/xo-reader/src/reader/progress_xs.cpp
+++ b/xo-reader/src/reader/progress_xs.cpp
@@ -118,7 +118,7 @@ namespace xo {
 
             case optype::op_assign:
             {
-                ref::brw<Variable> lhs = Variable::from(this->lhs_);
+                bp<Variable> lhs = Variable::from(this->lhs_);
 
                 if (!lhs) {
                     throw std::runtime_error
@@ -158,7 +158,7 @@ namespace xo {
         }
 
         void
-        progress_xs::on_expr(ref::brw<Expression> expr,
+        progress_xs::on_expr(bp<Expression> expr,
                              parserstatemachine * /*p_psm*/)
         {
             /* note: previous token probably an operator,
@@ -218,7 +218,7 @@ namespace xo {
         progress_xs::on_semicolon_token(const token_type & /*tk*/,
                                         parserstatemachine * p_psm)
         {
-            /* note: implementation parllels .on_rightparen_token() */
+            /* note: implementation parallels .on_rightparen_token() */
 
             constexpr bool c_debug_flag = true;
             scope log(XO_DEBUG(c_debug_flag));
diff --git a/xo-reader/src/reader/sequence_xs.cpp b/xo-reader/src/reader/sequence_xs.cpp
index 97aa0955..39e38365 100644
--- a/xo-reader/src/reader/sequence_xs.cpp
+++ b/xo-reader/src/reader/sequence_xs.cpp
@@ -32,7 +32,7 @@ namespace xo {
         {}
 
         void
-        sequence_xs::on_expr(ref::brw<Expression> expr,
+        sequence_xs::on_expr(bp<Expression> expr,
                              parserstatemachine * p_psm)
         {
              constexpr bool c_debug_flag = true;
@@ -67,7 +67,7 @@ namespace xo {
              * helpful to have nested seequence_xs that propagates '}'
              * instead of swallowing it.
              */
-            ref::brw<DefineExpr> def_expr = DefineExpr::from(expr);
+            bp<DefineExpr> def_expr = DefineExpr::from(expr);
 
             if (def_expr) {
                 /** nested_start: control returns via