_twM6B( nB )
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     Get the number of releases of a particular button since the last
     time this function (Interrupt 33h Function 6) was called. The
     number of releases ranges from 0 - 65,535 with no overflow. Use 0
     for the left button and 1 for the right.

     NOTE: Any call to Function 6 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 releases since the last call to this function.

       Example:

     // clear button release information for the left button.
     _twM6A()
     // do some things
     nNumberofreleases := _twM6B( 0 ) -> No. releases of left button.