fix 'append' in ISL+ to disallow a non-list last argument (merge to 4.2.2)

svn: r16166

collects/lang/private/intermediate-funs.ss

@@ -19,7 +19,7 @@
     )
 
    ("Lists"
-     (append ((listof any) ... -> (listof any))
+     ((intermediate-append append) ((listof any) ... -> (listof any))
        "to create a single list from several, by juxtaposition of the items"))
 
    ("Higher-Order Functions"

collects/lang/private/teachprims.ss

@@ -183,6 +183,14 @@ namespace.
     (check-last 'append (cons a (cons b x)))
     (apply append a b x)))
 
+(define-teach intermediate append
+  (lambda x
+    (if (null? x)
+        null
+        (begin
+          (check-last 'append x)
+          (apply append x)))))
+
 (define-teach beginner error
   (lambda (str)
     (unless  (string? str)
@@ -340,6 +348,7 @@ namespace.
  beginner-cons
  beginner-list*
  beginner-append
+ intermediate-append
  beginner-error
  beginner-struct?
  beginner-exit