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 RAT_EXIST() Determines if a mouse is available

 Returns
 <lExists> => mouse is available

 Syntax
 RAT_EXIST()

 Description
 This tells if a mouse is present, and mouse driver
 software is loaded. The mouse state is not changed.

 Examples
  if rat_exist()
    * do some mouse stuff
  else
    * don't do some mouse stuff
  endif

 Notes:
 Many mouse libraries use a call to Interrupt 33,
 service 0 to determine if a mouse is present. This has the
 unpleasant effect of  resetting the mouse to its defaults. This blows away
 mouse position,  cursor and other nifty things that may have been set.

 This function uses another means to determine if a
 mouse is present,  and does not reset the mouse or damage any mouse
 settings, thus you can use it with other mouse-oriented libraries
 without dire consequences.

 Source: S_MOUSE.ASM