twFileStack( nArea, nSelect )
 ----------------------------------------------------------------------------
     Save the current or other Alias, Workarea, index order, record
     number, filter condition and any relations set to an internal
     TSDWIN file stack.

       Arguments:

     nArea   - 'N' Work area to push. Defaults to the currently
                   selected area.

     nSelect - 'N' Area to select on exit. Defaults to the currently
                   selected area.

       Return:

     Numeric handle which is the position occupied in the internal
     TSDWIN file stack.

       Example:

     USE Clients INDEX Clients1 ALIAS Clients NEW
     USE Orders INDEX Orders1 ALIAS Orders NEW
     SELECT Clients
     SET RELATION TO CliNum INTO Orders

     nFHandle := twFileStack()