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.