_twM6A( nB )
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     Get the status of a mouse button released (0 = up, 1 = down). The
     button status is a single integer value. Bit 0 represents the left
     hand button and bit 1 the right hand button. The value of a bit is
     1 if the button is down and 0 if it is up.

       Arguments:

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

       Return:

     0 - button released.
     1 - button pressed.

       Example:

     nLeftButtonreleased := _twM6A( 0 )