NAME
        CBD_CURRENTWRITEID -- Determine the current write identifier.

   FUNCTION
        CBD_CURRENTWRITEID fills the io_ClipID with a clip identifier that
        can be compared with that of a post command: if greater than
        the post identifier then the post is obsolete and need never
        be satisfied.

   IO REQUEST
        io_Message      mn_ReplyPort set up
        io_Device       preset by opendevice
        io_Unit         preset by opendevice
        io_Command      CBD_CURRENTWRITEID

   RESULTS
        io_ClipID       the ClipID of the current write is set