added 'more: systems' doc

svn: r8341

original commit: b014545c7a7de0d7599a9383af830821f832ddb8

collects/mzlib/control.ss

@@ -1,6 +1,8 @@
 (module control mzscheme
 
-  (provide fcontrol %
+  (provide abort
+           
+           fcontrol %
 
            control prompt control-at prompt-at 
            ;; `-at' variations expect a prompt tag
@@ -16,6 +18,13 @@
 
            new-prompt set cupto)
 
+  ;; ----------------------------------------
+
+  (define (abort . vals)
+    (abort-current-continuation 
+     (default-continuation-prompt-tag)
+     (lambda () (apply values vals))))
+
   ;; ----------------------------------------
   ;; Sitaram, PLDI'93
   ;;  The `%' here is compable with Sitaram & Felleisen, LSC'90,