* added a sealed/freq field to the rib record. The idea is to

convert the rib data from lists to vectors once the rib is sealed
  (i.e. no more bindings will be added to it) and then sort it
  according to the frequency of references made to the individual
  bindings.

src/ikarus.syntax.ss

@@ -84,21 +84,23 @@
         [else (error 'gen-lexical "invalid arg ~s" sym)])))
   (define gen-label
     (lambda (_) (gensym)))
-  (define-record rib (sym* mark** label*))
+  (define-record rib (sym* mark** label* sealed/freq))
   (define make-full-rib
     (lambda (id* label*)
-      (make-rib (map id->sym id*) (map stx-mark* id*) label*)))
+      (make-rib (map id->sym id*) (map stx-mark* id*) label* #f)))
   (define make-empty-rib
     (lambda ()
-      (make-rib '() '() '())))
+      (make-rib '() '() '() #f)))
   (define extend-rib!
     (lambda (rib id label)
       (if (rib? rib)
           (let ([sym (id->sym id)] [mark* (stx-mark* id)])
+            (when (rib-sealed/freq rib)
+              (error 'extend-rib! "rib ~s is sealed" rib))
             (set-rib-sym*! rib (cons sym (rib-sym* rib)))
             (set-rib-mark**! rib (cons mark* (rib-mark** rib)))
             (set-rib-label*! rib (cons label (rib-label* rib))))
-          (error 'extend-rib! "~s is not an extensible rib" rib))))
+          (error 'extend-rib! "~s is not a rib" rib))))
   (module (make-stx stx? stx-expr stx-mark* stx-subst*)
     (define-record stx (expr mark* subst*)))
   (define datum->stx
@@ -280,7 +282,6 @@
       (cond
         [(imported-label->binding x)]
         [(assq x r) => cdr]
-        [(not x) (cons 'unbound #f)]
         [else (cons 'displaced-lexical #f)])))
   (define make-binding cons)
   (define binding-type car)
@@ -543,7 +544,7 @@
            (lambda (x)
              (let ([name (car x)] [label (cdr x)])
                (add-subst 
-                 (make-rib (list name) (list top-mark*) (list label))
+                 (make-rib (list name) (list top-mark*) (list label) #f)
                  (stx sym top-mark* '()))))]
           [else (stx sym top-mark* '())]))))
   ;;; macros