@ <row>, <col> WPROMPT <prompt> [MESSAGE <msg>] [COLOR <color>] ;
         [WHEN <when>] [ACTION <action>];
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Purpose: Do a TSDWIN @...PROMPT  for twMenuTo()

Parameters: row, col - 'N' Relative window coordinates

            prompt    - 'C' Clipper colour attribute.

            msg       - 'C or A' String or array of messages. May include
                        embedded colours.

            color     - 'C' Color in which to display the prompt.

            when      - 'B' Code  block to determine if item is
                        selectable.

            action    - 'B' optional code block to execute. If present,
                        the menu is not exited after execution.