twStopWatch( nStart, nStop )
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     Returns the number of clock ticks between nStart and nStop if
     nStart is greater than nStop.

       Arguments:

     nStart - 'N' Start time in seconds. A return from SECONDS().

     nStop  - 'N' Stop time in seconds.

       Return:

     Numeric value for nStop - nStart.

       Example:

     DO WHILE twStopWatch( nSecs, SECONDS()) <= nTime
     ENDDO