_twM5B( nB )
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     Get the number of presses of a particular button since the last
     time this function (Interrupt 33h Function 5) was called. The
     number of presses ranges from 0 - 65,535 with no overflow. Use 0
     for the left button and 1 for the right.

     NOTE: Any call to Function 5 resets the internal count.

       Arguments:

     nB - 'N' 0 if you want the status of the left hand button.
              1 for the right hand button.

       Return:

     Number of button presses  since the last call to this function.

       Example:

     // clear button press information for the left button.
     _twM5A()
     // do some things
     nNumberofPresses := _twM5B( 0 ) -> No. presses of left button.