diff --git a/include/xo/expression/Apply.hpp b/include/xo/expression/Apply.hpp
index b6db3a0c..1ed06e6b 100644
--- a/include/xo/expression/Apply.hpp
+++ b/include/xo/expression/Apply.hpp
@@ -63,6 +63,15 @@ namespace xo {
                 return n;
             }
 
+            virtual ref::rp<Expression> xform_layer(TransformFn xform_fn) override {
+                this->fn_ = fn_->xform_layer(xform_fn);
+
+                for (auto & arg : argv_)
+                    arg = arg->xform_layer(xform_fn);
+
+                return xform_fn(this);
+            }
+
             virtual void attach_envs(ref::brw<Environment> p) override {
                 fn_->attach_envs(p);
 
diff --git a/include/xo/expression/Constant.hpp b/include/xo/expression/Constant.hpp
index 50f4c2ba..ffc15967 100644
--- a/include/xo/expression/Constant.hpp
+++ b/include/xo/expression/Constant.hpp
@@ -56,6 +56,10 @@ namespace xo {
                 return 1;
             }
 
+            virtual ref::rp<Expression> xform_layer(TransformFn xform_fn) override {
+                return xform_fn(this);
+            }
+
             virtual void display(std::ostream & os) const override {
                 os << "<Constant"
                    << xtag("type", value_td_->short_name())
diff --git a/include/xo/expression/Expression.hpp b/include/xo/expression/Expression.hpp
index d23a412e..84248555 100644
--- a/include/xo/expression/Expression.hpp
+++ b/include/xo/expression/Expression.hpp
@@ -35,7 +35,10 @@ namespace xo {
          **/
         class Expression : public ref::Refcount {
         public:
-            using VisitFn = std::function<void (ref::brw<Expression>)>;
+            using VisitFn = std::function
+                <void (ref::brw<Expression>)>;
+            using TransformFn = std::function
+                <ref::rp<Expression> (ref::brw<Expression>)>;
             using TypeDescr = xo::reflect::TypeDescr;
 
         public:
@@ -58,6 +61,9 @@ namespace xo {
              **/
             virtual std::size_t visit_preorder(VisitFn visitor_fn) = 0;
 
+            /** traverse ast @ref visit_preorder but do not visit Lambdas **/
+            virtual ref::rp<Expression> xform_layer(TransformFn visitor_fn) = 0;
+
             /** attach an environment to each lambda expression X in this subtree,
              *  that will:
              *  - resolve names matching X's arguments (formal parameters) to
diff --git a/include/xo/expression/IfExpr.hpp b/include/xo/expression/IfExpr.hpp
index fc8204a7..8091481e 100644
--- a/include/xo/expression/IfExpr.hpp
+++ b/include/xo/expression/IfExpr.hpp
@@ -60,12 +60,21 @@ namespace xo {
 
                 visitor_fn(this);
 
+                n += this->test_->visit_preorder(visitor_fn);
                 n += this->when_true_->visit_preorder(visitor_fn);
                 n += this->when_false_->visit_preorder(visitor_fn);
 
                 return n;
             }
 
+            virtual ref::rp<Expression> xform_layer(TransformFn xform_fn) override {
+                this->test_ = this->test_->xform_layer(xform_fn);
+                this->when_true_ = this->when_true_->xform_layer(xform_fn);
+                this->when_false_= this->when_false_->xform_layer(xform_fn);
+
+                return xform_fn(this);
+            }
+
             virtual void attach_envs(ref::brw<Environment> p) override {
                 test_->attach_envs(p);
                 when_true_->attach_envs(p);
diff --git a/include/xo/expression/Lambda.hpp b/include/xo/expression/Lambda.hpp
index 619913a1..18039982 100644
--- a/include/xo/expression/Lambda.hpp
+++ b/include/xo/expression/Lambda.hpp
@@ -68,6 +68,11 @@ namespace xo {
                 return n;
             }
 
+            virtual ref::rp<Expression> xform_layer(TransformFn /*xform_fn*/) override {
+                /* a layer is bounded by lambdas,  don't enter them */
+                return this;
+            }
+
             virtual void attach_envs(ref::brw<Environment> p) override {
                 local_env_->assign_parent(p);
             }
diff --git a/include/xo/expression/PrimitiveInterface.hpp b/include/xo/expression/PrimitiveInterface.hpp
index c52eb979..70080607 100644
--- a/include/xo/expression/PrimitiveInterface.hpp
+++ b/include/xo/expression/PrimitiveInterface.hpp
@@ -57,6 +57,10 @@ namespace xo {
                 return 1;
             }
 
+            virtual ref::rp<Expression> xform_layer(TransformFn xform_fn) override {
+                return xform_fn(this);
+            }
+
             virtual void attach_envs(ref::brw<Environment> /*p*/) override {}
 
         private:
diff --git a/include/xo/expression/Variable.hpp b/include/xo/expression/Variable.hpp
index 3f45677e..713da66f 100644
--- a/include/xo/expression/Variable.hpp
+++ b/include/xo/expression/Variable.hpp
@@ -43,6 +43,10 @@ namespace xo {
                 return 1;
             }
 
+            virtual ref::rp<Expression> xform_layer(TransformFn xform_fn) override {
+                return xform_fn(this);
+            }
+
             virtual void attach_envs(ref::brw<Environment> /*p*/) override;
 
             virtual void display(std::ostream & os) const override;
diff --git a/src/expression/Lambda.cpp b/src/expression/Lambda.cpp
index 2f84b178..b2e4d1f3 100644
--- a/src/expression/Lambda.cpp
+++ b/src/expression/Lambda.cpp
@@ -4,6 +4,7 @@
 #include "xo/reflect/TypeDescr.hpp"
 #include "xo/reflect/function/FunctionTdx.hpp"
 #include "xo/indentlog/print/vector.hpp"
+#include <map>
 
 namespace xo {
     using xo::reflect::TypeDescrBase;
@@ -25,10 +26,12 @@ namespace xo {
              **/
 
             std::vector<TypeDescr> arg_td_v;
-            arg_td_v.reserve(argv.size());
+            {
+                arg_td_v.reserve(argv.size());
 
-            for (const auto & arg : argv) {
-                arg_td_v.push_back(arg->valuetype());
+                for (const auto & arg : argv) {
+                    arg_td_v.push_back(arg->valuetype());
+                }
             }
 
             auto function_info
@@ -85,6 +88,30 @@ namespace xo {
             }
             ss << ")";
 
+            /* regularize local_env+body:  make sure exactly one instance
+             * (i.e. with object identity) of a Variable appears
+             * within one layer of a lambda body.
+             *
+             * Here 'layer' means excluding appearance in any nested lambdas
+             * (i.e. whether or not such appearance would resolve to the same
+             * memory location).
+             *
+             * Motivation is to unify Variables that would use the same
+             * binding_path to resolve their runtime location.
+             */
+            {
+                std::map<std::string, rp<Variable>> var_map;
+
+                for (const auto & arg : local_env_->argv()) {
+                    /* each arg name can appear at most once
+                     * in a particular lambda's parameter list
+                     */
+                    assert(var_map.find(arg->name()) == var_map.end());
+
+                    var_map[arg->name()] = arg;
+                }
+            }
+
             this->type_str_ = ss.str();
             this->free_var_set_ = this->calc_free_variables();