Added remf to unstable/list.

Signed-off-by: Jay McCarthy <jay@racket-lang.org>

collects/tests/unstable/list.rkt

@@ -0,0 +1,9 @@
+#lang scheme
+(require unstable/list)
+(require tests/eli-tester)
+(test
+ (remf positive? '()) => '()
+ (remf positive? '(1 -2 3 4 -5)) => '(-2 3 4 -5)
+ (remf even? '(1 -2 3 4 -5)) => '(1 3 4 -5)
+ (remf (λ (x) #f) '(1 -2 3 4 -5)) => '(1 -2 3 4 -5))
+

collects/unstable/list.rkt

@@ -112,3 +112,13 @@
 
 (provide map/values)
 
+;; dvanhorn added:
+
+(define (remf f ls)
+  (cond [(null? ls) '()]
+        [(f (car ls)) (cdr ls)]
+        [else 
+         (cons (car ls)
+               (remf f (cdr ls)))]))
+
+(provide/contract [remf (-> procedure? list? list?)])

collects/unstable/scribblings/list.scrbl

@@ -103,3 +103,18 @@ Produces lists of the respective values of @racket[f] applied to the elements in
 
 }
 
+@addition{David Van Horn}
+
+@defproc[(remf [pred procedure?]
+               [lst list?])
+         list?]{
+Returns a list that is like @racket[lst], omitting the first element of @racket[lst] 
+for which @racket[pred] produces a true value.
+
+@defexamples[
+#:eval the-eval
+(remf negative? '(1 -2 3 4 -5))
+]
+
+}
+