Stack class structure
~~~~~~~~~~~~~~~~~~~~~~~
create class Stack
  export:
  var Data    //{}
  method New=StackNew           //o:New() --> self
  method Init=StackInit         //o:Init() --> true
  method Push=StackPush         //o:Push(val) --> xVal
  method Pop=StackPop           //o:Pop() --> xVal
  method Top=StackTop           //o:Top() --> xVal
  method IsEmpty=StackIsEmpty   //o:IsEmpty() --> true/false
  method Done=StackDone         //o:Done() --> true
  endclass