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	the post identifier then the post is obsolete and need never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