adjust plug to be parsimonious and actually shortcircuit

when one hole has been found

This improves the lambdajs model example's running time, presumably
because the hole is generally found near the "beginning" of the
term

collects/redex/private/matcher.rkt

@@ -1779,21 +1779,26 @@ See match-a-pattern.rkt for more details
 (define (reverse-context x) (reverse x))
 (define (build-nested-context c1 c2) 
   (plug c1 c2))
+
 (define (plug exp hole-stuff)
-  (let ([done? #f])
-    (let loop ([exp exp])
-      (cond
-        [(pair? exp) 
-         (cons (loop (car exp))
-               (loop (cdr exp)))]
-        [(eq? the-not-hole exp)
-         the-not-hole]
-        [(eq? the-hole exp)
-         (if done?
-             exp
-             (begin (set! done? #t)
-                    hole-stuff))]
-        [else exp]))))
+  (let loop ([exp exp])
+    (cond
+      [(pair? exp) 
+       (define old-car (car exp))
+       (define new-car (loop old-car))
+       (cond
+         [(eq? new-car old-car)
+          (define old-cdr (cdr exp))
+          (define new-cdr (loop old-cdr))
+          (if (eq? new-cdr old-cdr)
+              exp
+              (cons new-car new-cdr))]
+         [else (cons new-car (cdr exp))])]
+      [(eq? the-not-hole exp)
+       the-not-hole]
+      [(eq? the-hole exp)
+       hole-stuff]
+      [else exp])))
 
 ;;
 ;; end context adt