diff --git a/collects/macro-debugger/model/reductions-engine.ss b/collects/macro-debugger/model/reductions-engine.ss
index 6335540..d5b2f2a 100644
--- a/collects/macro-debugger/model/reductions-engine.ss
+++ b/collects/macro-debugger/model/reductions-engine.ss
@@ -44,15 +44,13 @@
     (syntax-rules ()
       [(with-new-local-context e . body)
        (parameterize ([big-context
-                       (cons (cons (current-derivation)
-                                   (cons (list e)
-                                         (context)))
+                       (cons (make-bigframe (current-derivation) (context) (list e) e)
                              (big-context))]
                       [context null])
          . body)]))
-  
+
   ;; -----------------------------------
-  
+
   ;; CC
   ;; the context constructor
   (define-syntax (CC stx)
@@ -66,7 +64,7 @@
     (syntax-rules ()
       [(R form pattern . clauses)
        (R** #f _ [#:set-syntax form] [#:pattern pattern] . clauses)]))
-  
+
   (define-syntax (R** stx)
     (syntax-case stx (! @ List Block =>)
       [(R** form-var pattern)
diff --git a/collects/macro-debugger/model/steps.ss b/collects/macro-debugger/model/steps.ss
index 80906ad..18c3478 100644
--- a/collects/macro-debugger/model/steps.ss
+++ b/collects/macro-debugger/model/steps.ss
@@ -5,13 +5,14 @@
   ;; A ReductionSequence is a (list-of Reduction)
 
   ;; A ProtoStep is (make-protostep Derivation BigContext StepType Context)
-  
+
   ;; A Context is a list of Frames
   ;; A Frame is (syntax -> syntax)
 
-  ;; A BigContext is (list-of (cons Derivation (cons Syntaxes Syntax)))
-  ;;   local expansion contexts: deriv, foci, term
-  
+  ;; A BigContext is (list-of BigFrame)
+  ;; A BigFrame is (make-bigframe Derivation Context Syntaxes Syntax)
+  (define-struct bigframe (deriv ctx foci e))
+
   ;; A Reduction is one of 
   ;;   - (make-step ... Syntaxes Syntaxes Syntax Syntax)
   ;;   - (make-misstep ... Syntax Syntax Exception)
@@ -36,6 +37,9 @@
   (define (misstep-term1 s)
     (context-fill (protostep-ctx s) (misstep-e1 s)))
 
+  (define (bigframe-term bf)
+    (context-fill (bigframe-ctx bf) (bigframe-e bf)))
+  
   ;; A StepType is a simple in the following alist.
 
   (define step-type-meanings