correction

This is a correction to the talk I gave in Toronto on the 24th of September. In the definition of "principal computad" (on page 8 of the slides, for those who have copies of the slides) a clause was omitted. The definition should read as follows (clause (c) was missing): The computad A is *principal* if (a) A is finite, (b) there is a unique indeterminate of maximal dimension in A, say x; and (c) there is no proper subcomputad of A containing x. I add that there was another error in the slides, although it was caught during the talk and corrected on the projected slide (but not on the copies). On page 11 of the slides, "fibration" should be "trivial fibration". Michael Makkai