UCD question: how to specify reference to a flat field image?

[Brian Thomas]

	Hi,

	Here's a question for the UCD mavens on the list; my apologies in advance if
	this was answered elsewhere  (I guess I missed it..)

	How does one specify "a flat-field calibration file" using UCD 1.23? I see that the
	'meta.file' and 'obs.calib.flat' UCDs both exist, however, they are both marked as
	'secondary', so neither may be the first in a composed word, and apparently cant
	be combined.

	What should I do here?

	-brian