NAME
    CBD_CURRENTREADID - Determine the current read identifier.

FUNCTION
    CBD_CURRENTREADID 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 data held privately by an
    application is not valid for its own pasting.

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

RESULTS
    io_ClipID       the ClipID of the current write is set