datatype message = 
    IsNull 
  | Add of string 
  | HasMore 
  | Remove 
  | GetData 
  | GetLink;

datatype response = 
    Pred of bool 
  | Data of string 
  | Removed of (message -> response) * string 
  | Object of message -> response;

fun root _ = Pred false;

fun null IsNull = Pred true
  | null message = root message;

fun node data link GetData = Data data
  | node data link GetLink = Object link
  | node _ _ message = root message;

fun stack top HasMore = 
      let val Pred(p) = top IsNull in Pred(not p) end
  | stack top (Add data) = Object(stack (node data top))
  | stack top Remove =
      let
        val Object(next) = top GetLink
        val Data(data) = top GetData
      in 
        Removed(stack next, data)
      end
  | stack _ message = root message;