Cursor class structure:
~~~~~~~~~~~~~~~~~~~~~~~~
create class Cursor from Loc
  export:
  var Size    //SC_NONE
  method New=CursorNew      //o:New() --> self
  method Init=CursorInit    //o:Init(Row,Col,Size) --> true
  method Get=CursorGet      //o:Get() --> true
  method Set=CursorSet      //o:Set() --> true
  endclass