Add `stepper-skipto/discard' property.

Like stepper-skipto, but discards the context.  This is for the
contracts implementation in the DMdA teaching languages.

svn: r12078

collects/stepper/internal-docs.txt

@@ -202,6 +202,16 @@ Where it's used: the stepper-skipto label is used by the 2nd-pass
 macro-labeler and the annotator.  Both are in annotate.ss.  In addition
 to skipping inward, a stepper hint
 
+stepper-skipto/discard :
+
+ This is like stepper-skipto, except that it makes the stepper
+ replace the expression the property is attached to by the
+ subexpression indicated by its value.
+
+ (This is used in the contracts implementation for "Die Macht der
+ Abstraktion", where procedures are wrapped in a contract-checking
+ context that has no impact on the reduction semantics.)
+
 stepper-else :
  [ #t ] : Initially applied to the 'true' that the cond macro
   replaces a beginner's 'else' with, it is later transferred

collects/stepper/private/annotate.ss

@@ -387,13 +387,17 @@
                . -> . (vector/p syntax? binding-set?))
     (lambda (exp tail-bound pre-break? procedure-name-info)
       
-      (cond [(stepper-syntax-property exp 'stepper-skipto)
+      (cond [(cond
+	      ((stepper-syntax-property exp 'stepper-skipto) 'rebuild)
+	      ((stepper-syntax-property exp 'stepper-skipto/discard) 'discard)
+	      (else #f))
+	     => (lambda (traversal)
 		  (let* ([free-vars-captured #f] ; this will be set!'ed
-                    ;[dont-care (printf "expr: ~a\nskipto: ~a\n" expr (stepper-syntax-property expr 'stepper-skipto))]
-                    ; WARNING! I depend on the order of evaluation in application arguments here:
+			 ;;[dont-care (printf "expr: ~a\nskipto: ~a\n" expr (stepper-syntax-property expr 'stepper-skipto))]
+			 ;; WARNING! I depend on the order of evaluation in application arguments here:
 			 [annotated (skipto/auto
 				     exp
-                                'rebuild
+				     traversal
 				     (lambda (subterm)
 				       (let*-2vals ([(stx free-vars) (annotate/inner subterm tail-bound pre-break? procedure-name-info)])
 						   (set! free-vars-captured free-vars)
@@ -401,7 +405,7 @@
 		    (2vals (wcm-wrap
 			    skipto-mark
 			    annotated)
-                      free-vars-captured))]
+			   free-vars-captured)))]
             
             [(stepper-syntax-property exp 'stepper-skip-completely)
              (2vals (wcm-wrap 13 exp) null)]

collects/stepper/private/shared.ss

@@ -481,7 +481,8 @@
   ;; traversal argument is 'discard, the result of the transformation is the
   ;; result of this function
   (define (skipto/auto stx traversal transformer)
-    (cond [(stepper-syntax-property stx 'stepper-skipto)
+    (cond [(or (stepper-syntax-property stx 'stepper-skipto)
+	       (stepper-syntax-property stx 'stepper-skipto/discard))
            =>
            (cut update <> stx (cut skipto/auto <> traversal transformer) traversal)]
           [else (transformer stx)]))