diff --git a/include/xo/reader/parserstatemachine.hpp b/include/xo/reader/parserstatemachine.hpp
index c7ca130c..e39021f3 100644
--- a/include/xo/reader/parserstatemachine.hpp
+++ b/include/xo/reader/parserstatemachine.hpp
@@ -6,6 +6,7 @@
 #pragma once
 
 #include "exprstate.hpp"
+#include "envframestack.hpp"
 
 namespace xo {
     namespace scm {
@@ -22,15 +23,24 @@ namespace xo {
 
         public:
             parserstatemachine(exprstatestack * p_stack,
+                               envframestack * p_env_stack,
                                rp<Expression> * p_emit_expr)
-                : p_stack_{p_stack}, p_emit_expr_{p_emit_expr} {}
+                : p_stack_{p_stack},
+                  p_env_stack_{p_env_stack},
+                  p_emit_expr_{p_emit_expr} {}
 
             std::unique_ptr<exprstate> pop_exprstate();
             exprstate & top_exprstate();
             void push_exprstate(std::unique_ptr<exprstate> x);
 
         public:
+            /** stack of incomplete parser work.
+             *  generally speaking, push when to start new work for nested content;
+             *  pop when work complete
+             **/
             exprstatestack * p_stack_;
+            /** stack of environment frames, one for each enclosing lambda **/
+            envframestack * p_env_stack_;
             rp<Expression> * p_emit_expr_;
         };
     } /*namespace scm*/
diff --git a/src/reader/parser.cpp b/src/reader/parser.cpp
index 9301f1df..e71deb40 100644
--- a/src/reader/parser.cpp
+++ b/src/reader/parser.cpp
@@ -33,6 +33,7 @@ namespace xo {
         parser::begin_translation_unit() {
             /* note: not using emit expr here */
             parserstatemachine psm(&xs_stack_,
+                                   &env_stack_,
                                    nullptr /*p_emit_expr*/);
 
             exprseq_xs::start(&psm);
@@ -55,7 +56,7 @@ namespace xo {
 
             rp<Expression> retval;
 
-            parserstatemachine psm(&xs_stack_, &retval);
+            parserstatemachine psm(&xs_stack_, &env_stack_, &retval);
 
             xs_stack_.top_exprstate().on_input(tk, &psm);