Single factor from interaction.

I have a vague recollection of seeing reference, fairly recently, to
a function that forms a single factor which ``codes'' the interaction
between two (or more?) factors.  Do I recollect correctly?  It would
be easy enough to roll one's own, but if there is an existing
function it probably does a much cleverer job than I would do.

				cheers,

					Rolf Turner
					rolf at math.unb.ca