NAME
        CBD_POST -- Post availability of a clip to the clipboard.

   FUNCTION
        Indicate to the clipboard device that data is available for
        use by accessors of the clipboard.  This is intended to be
        used when a cut is large, in a private data format, and/or
        changing frequently, and it thus makes sense to avoid
        converting it to an IFF form and writing it to the clipboard
        unless another application wants it.  The post provides a
        message port to which the clipboard device will send a satisfy
        message if the data is required.

        If the satisfy message is received, the write associated with
        the post must be performed.  The act of writing the clip
        indicates that the message has been received: it may then be
        re-used by the clipboard device, and so must actually be
        removed from the satisfy message port so that the port is not
        corrupted.

        If the application wishes to determine if a post it has
        performed is still the current clip, it should check the
        post's io_ClipID with that returned by the cbd_currentreadid
        command.  If the current read io_ClipID is greater, the clip
        is not still current.

        If an application has a pending post and wishes to determine
        if it should satisfy it (e.g. before it exits), it should
        check the post's io_ClipID with that returned by the
        cbd_currentwriteid command.  if the current write io_clipid is
        greater, there is no need to satisfy the post.

   IO REQUEST
        io_Message      mn_ReplyPort set up
        io_Device       preset by opendevice
        io_Unit         preset by opendevice
        io_Command      CBD_POST
        io_Data         pointer to satisfy message port
        io_ClipID       zero

   RESULTS
        io_Error        non-zero if an error occurred
        io_ClipID       the clip ID assigned to this post, to be used
                        in the write command if this is satisfied