NAME
        CBD_CHANGEHOOK -- Add or remove a clip change hook

   FUNCTION
        CBD_CHANGEHOOK allows specification of a hook to be called
        when the data on the clipboard has changed.

   IO REQUEST
        io_Message      mn_ReplyPort set up
        io_Device       preset by opendevice
        io_Unit         preset by opendevice
        io_Command      CBD_CHANGEHOOK
        io_Length -     0 to remove, 1 to install this hook
        io_Data -       struct hook *, the clip change hook

   HOOK ENVIRONMENT
       hook message - a cliphookmsg, as defined in devices/clipboard.h
           chm_Type - 0, indicating that the message has the
               following fields:
           chm_ClipID - the clip ID of the clip triggering the change
       hook object - io_Unit