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    the post identifier then the post is obsolete and need never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