redex: elminate another #f/failure confusion

(cherry picked from commit 468d2b192e)

collects/redex/private/search.rkt

@@ -186,7 +186,7 @@
             [(cons (eqn lhs rhs) rest)
              (eqn lhs rhs)
              (define u-res (unify lhs rhs e lang))
-             (and u-res
+             (and (not-failed? u-res)
                   (loop (p*e-e u-res) rest))]))]
        [else #f])]))
            

collects/redex/tests/gen-test.rkt

@@ -15,8 +15,9 @@
 (define-syntax-rule (is-false e)
   (test e #f))
 
+(define-language L0)
+
 (let ()
-  (define-language L0)
   
   (test (check-dq (dq '() (list `a `a)) (make-hash) L0 (hash))
               #f)
@@ -590,7 +591,6 @@
         (g)))))
 
 (let ()
-  (define-language L0)
   (define-relation L0
     [(a any)])
   (define-relation L0
@@ -629,9 +629,6 @@
 
 (let ()
   
-  
-  (define-language L0)
-  
   (define-relation L0
     [(R number)
      number]
@@ -677,7 +674,6 @@
   (is-not-false (generate-term L0 #:satisfying (J ((#t) (2))) 5)))
 
 (let ()
-  (define-language L0)
   
   (define-metafunction L0
     [(f (any_1 any_2))
@@ -713,7 +709,6 @@
        [else #t]))))
 
 (let ()
-  (define-language L0)
   
   (define-relation L0
     [(R (#f #f #f) 3)]
@@ -754,4 +749,26 @@
     #:satisfying
     (R #f any)
     +inf.0))
-  )
+  )
+
+(let ()
+  
+  (define-relation L0
+    [(not-mem any_1 (any_2 any_3))
+     (not-mem any_1 any_3)
+     (where (any_!_4 any_!_4) (any_1 any_2))]
+    [(not-mem any_1 ())])
+  
+  (is-not-false
+   (generate-term
+    L0
+    #:satisfying
+    (not-mem d (a (b (c ()))))
+    +inf.0))
+  
+  (is-false
+   (generate-term
+    L0
+    #:satisfying
+    (not-mem b (a (b (c ()))))
+    +inf.0)))