added the dllist to the tree

original commit: 0eecce89853d2fd9aa18765f440113bf8cafcdbf

collects/embedded-gui/private/dllist.ss

@@ -0,0 +1,52 @@
+(module dllist mzscheme
+  
+  (require
+   (lib "class.ss")
+   (lib "list.ss")
+   (lib "etc.ss")
+   (lib "framework.ss" "framework")
+   "interface.ss")
+  
+  (provide dllist-mixin dllist% head% tail%)
+  
+  (define dllist-mixin
+    (mixin () (dllist<%>)
+      (field [nextf #f] [prevf #f])
+      (define/public next
+        (case-lambda
+          [(n) (set! nextf n)]
+          [() nextf]))
+      (define/public prev
+        (case-lambda
+          [(p) (set! prevf p)]
+          [() prevf]))
+      (define/public (for-each f . lol)
+        (apply f this (map first lol))
+        (send/apply nextf for-each f (map rest lol)))
+      (define/public (map-to-list f)
+        (cons (f this) (send nextf map-to-list f)))
+      (super-new)))
+  
+  (define dllist% (dllist-mixin object%))
+  
+  (define head%
+    (class* dllist% (dllist<%>)
+      (inherit next)
+      (define/override prev
+        (opt-lambda ((p #f))
+          (error 'prev "Head has no prev")))
+      (define/override (for-each f . lol)
+        (send/apply (next) for-each f lol))
+      (define/override (map-to-list f)
+        (send (next) map-to-list f))
+      (super-new)))
+  
+  (define tail%
+    (class* dllist% (dllist<%>)
+      (define/override next
+        (opt-lambda ((n #f))
+          (error 'next "Tail has no next")))
+      (define/override (for-each f . lol) (void))
+      (define/override (map-to-list f) empty)
+      (super-new)))
+  )