twMButtonDown( nButton )
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     Get mouse button status. This function determines the status of
     the left and right hand mouse buttons. 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:

     None.

       Return:

     0 - no buttons down.
     1 - left button down.
     2 - right button down.
     3 - both buttons down.

       Example:

     IF ( twMButtonDown() == 1 )
        // do something
     ENDIF