NAME
	IND_ADDHANDLER -- Add an input handler to the device

   FUNCTION
	Add a function to the list of functions called to handle
	input events generated by this device.  The function is called
	as
	    newInputEvents = Handler(inputEvents, handlerData);
	    D0			     A0		  A1

   IO REQUEST
	io_Message	mn_ReplyPort set
	io_Device	preset by OpenDevice
	io_Unit		preset by OpenDevice
	io_Command	IND_ADDHANDLER
	io_Data		a pointer to an interrupt structure.
	    is_Data 	the handlerData pointer described above
	    is_Code	the Handler function address

   NOTES
	The interrupt structure is kept by the input device until a
	RemHandler command is satisfied for it.