Constants

      MouseLeftButton  =    0
      MouseRightButton =    1
      MouseMiddleButton=    2

    These are used to determine which mouse button was pressed.

      MouseGranularity =    8

    The mouse functions  return  mouse  coordinates  in pixels and always
    assume that the font is 8x8 in  size.   Thus any coordinates received
    by a mouse function should be divided by 8 (MouseGranularity).

      SpecialNIL       =    Ptr($FFFF,$FFFF)

    NIL cannot be used to check  whether  the  OldExitProc variables have
    been used or not so I had to define my own.