sts.i4.v = seqdat_update(update_what.i4.v, new_val.g.r)

	Update SEQUENCER/C49 data for subsequent reading with seqdat_read.
	Previous call to seqdat_set_mode required.  Arguments are as follows.

	update_what	see auxlib include file for defined constants
			that specify the data to be updated

	new_val		new data to save; this will have different (or no)
			meaning, depending on 'update_what' argument

	Returns 0 for success, else a nonzero error code