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        Set alternate mouse user handler

        Entry:
          AX = 0018h
          CX = call mask
               bit 0 call if alt key pressed during event
               bit 1 call if ctrl key pressed during event
               bit 2 call if shift button pressed during event
               bit 3 call if right button released
               bit 4 call if right button pressed
               bit 5 call if left button released
               bit 6 call if left button pressed
               bit 7 call if mouse moves
          BX(???):DX = address of FAR routine

        Note:   When the subroutine is called, it is passed the following
                values:

          AX = condition mask (same bit assignments as call mask)
          BX = button state
          CX = cursor column
          DX = cursor row
          DI = horizontal mickey count
          SI = vertical mickey count

        Note:   Up to three handlers can be defined by separate calls to
                this function