_twM11D()
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     Get vertical direction the mouse has moved since the last call to
     this function. The return value is in 'Mickeys' (0.5mm or 0.02
     inches.) The return value ranges from -32768 to 32768.

       Arguments:

     None.

       Return:

     Number of mickeys moved vertically since the last call to this
     function.

       Example:

     nVertMickeys := _twM11D()