add check-redundancy, a parameter that causes redex to print some

debugging information about ambiguous matching (when the ambiguity
matters)

collects/redex/private/matcher.rkt

@@ -1624,7 +1624,18 @@ See match-a-pattern.rkt for more details
           [(null? rhss) 
            (if (null? ans)
                #f
-               ans)]
+               (begin
+                 (when (check-redudancy)
+                   (let ([rd (remove-duplicates ans)])
+                     (unless (= (length rd) (length ans))
+                       (eprintf "found redundancy when matching the non-terminal ~s against:\n~s~a"
+                                nt
+                                term
+                                (apply
+                                 string-append
+                                 (map (λ (x) (format "\n  ~s" x))
+                                      ans))))))
+                 ans))]
           [else
            (let ([mth (call-nt-proc/bindings (car rhss) term hole-info)])
              (cond
@@ -1644,6 +1655,8 @@ See match-a-pattern.rkt for more details
            (or (call-nt-proc/bindings (car rhss) term hole-info)
                (loop (cdr rhss)))]))))
 
+(define check-redudancy (make-parameter #f))
+
 (define (match-nt/boolean list-rhs non-list-rhs nt term)
   (let loop ([rhss (if (or (null? term) (pair? term))
                        list-rhs
@@ -1956,4 +1969,5 @@ See match-a-pattern.rkt for more details
          the-not-hole the-hole hole?
          rewrite-ellipses
          build-compatible-context-language
-         caching-enabled?)
+         caching-enabled?
+         check-redudancy)

collects/redex/redex.scrbl

@@ -389,7 +389,17 @@ nested lists.
 }
 
 @defproc[(set-cache-size! [size positive-integer?]) void?]{
-Changes the size of the per-pattern and per-metafunction caches. The default size is @racket[350].
+Changes the size of the per-pattern and per-metafunction caches.
+
+The default size is @racket[350].
+}
+
+@defparam[check-redudancy check? boolean?]{
+  Ambiguous patterns can slow down
+  Redex's pattern matching implementation significantly. To help debug
+  such performance issues, set the @racket[check-redundancy]
+  parameter to @racket[#t]. This causes Redex to, at runtime,
+  report any redundant matches that it encounters.
 }
 
 @section{Terms}

collects/redex/reduction-semantics.rkt

@@ -33,7 +33,8 @@
          judgment-holds
          in-domain?
          caching-enabled?
-         make-coverage)
+         make-coverage
+         check-redudancy)
 
 (provide (rename-out [test-match redex-match])
          term-match