Fixed `stream-add-between'.

Don't put an extra seperator element at the end of the stream.  Brings
it in-line with the list version (`add-between') and sequence
version (`sequence-add-between').  Includes a test.

collects/racket/stream.rkt

@@ -152,12 +152,15 @@
       (make-do-stream (lambda () (force!) empty?)
                       (lambda () (force!) fst)
                       (lambda () (force!) rst)))]))
-  
+
 (define (stream-add-between s e)
   (unless (stream? s)
     (raise-type-error 'stream-add-between "stream" s))
-  (let loop ([s s])
-    (cond
-     [(stream-empty? s) empty-stream]
-     [else (stream-cons (stream-first s)
-                        (stream-cons e (loop (stream-rest s))))])))
+  (if (stream-empty? s)
+      empty-stream
+      (stream-cons
+       (stream-first s)
+       (let loop ([s (stream-rest s)])
+         (cond [(stream-empty? s) empty-stream]
+               [else (stream-cons e (stream-cons (stream-first s)
+                                                 (loop (stream-rest s))))])))))

collects/tests/racket/sequence.rktl

@@ -1,4 +1,3 @@
-
 (load-relative "loadtest.rktl")
 
 (Section 'sequence)
@@ -108,6 +107,8 @@
         (sequence-ref (sequence-add-between (in-naturals) #t) 2))
   (test #t 'sequence-add-between
         (sequence-ref (sequence-add-between (in-naturals) #t) 3))
+  (test 3 'sequence-add-between
+        (sequence-length (sequence-add-between (in-range 2) #t)))
 
   (arity-test sequence-count 2 2)
   (test 0 'sequence-count (sequence-count even? empty-sequence))