--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/demo/more/dlist.ur	Tue Sep 08 07:48:57 2009 -0400
@@ -0,0 +1,88 @@
+datatype dlist'' t =
+         Nil
+       | Cons of t * source (dlist'' t)
+
+datatype dlist' t =
+         Empty
+       | Nonempty of { Head : dlist'' t, Tail : source (source (dlist'' t)) }
+
+con dlist t = source (dlist' t)
+
+type position = transaction unit
+
+fun headPos [t] (dl : dlist t) =
+    dl' <- get dl;
+    case dl' of
+        Nonempty { Head = Cons (_, tl), Tail = tl' } =>
+        cur <- get tl;
+        set dl (case cur of
+                    Nil => Empty
+                  | _ => Nonempty {Head = cur, Tail = tl'})
+      | _ => return ()
+
+fun tailPos [t] (cur : source (dlist'' t)) new tail =
+    new' <- get new;
+    set cur new';
+
+    case new' of
+        Nil => set tail cur
+      | _ => return ()
+
+val create = fn [t] => source Empty
+
+fun clear [t] (s : dlist t) = set s Empty
+
+fun append [t] dl v =
+    dl' <- get dl;
+    case dl' of
+        Empty =>
+        tl <- source Nil;
+        tl' <- source tl;
+        set dl (Nonempty {Head = Cons (v, tl), Tail = tl'});
+        return (headPos dl)
+                
+      | Nonempty {Tail = tl, ...} =>
+        cur <- get tl;
+        new <- source Nil;
+        set cur (Cons (v, new));
+        set tl new;
+        return (tailPos cur new tl)
+
+fun render [ctx] [ctx ~ body] [t] f dl = <xml>
+  <dyn signal={dl' <- signal dl;
+               return (case dl' of
+                           Empty => <xml/>
+                         | Nonempty {Head = hd, Tail = tlTop} => 
+                           let
+                               fun render' prev dl'' =
+                                   case dl'' of
+                                       Nil => <xml/>
+                                     | Cons (v, tl) =>
+                                       let
+                                           val pos = case prev of
+                                                         None => headPos dl
+                                                       | Some prev => tailPos prev tl tlTop
+                                       in
+                                           <xml>{f v pos}<dyn signal={tl' <- signal tl;
+                                                                      return (render' (Some tl) tl')}/></xml>
+                                       end
+                           in
+                               render' None hd
+                           end)}/>
+</xml>
+
+fun delete pos = pos
+
+fun elements' [t] (dl'' : dlist'' t) =
+    case dl'' of
+        Nil => return []
+      | Cons (x, dl'') =>
+        dl'' <- signal dl'';
+        tl <- elements' dl'';
+        return (x :: tl)
+
+fun elements [t] (dl : dlist t) =
+    dl' <- signal dl;
+    case dl' of
+        Empty => return []
+      | Nonempty {Head = hd, ...} => elements' hd