Re: Pratt slices

From: Dr. P.T. Johnstone <P.T.Johnstone@dpmms.cam.ac.uk> So we ought surely to be looking for a common generalization, not of toposes and abelian categories, but of (pre)toposes and affine categories in Aurelio's sense.

In that context let me rephrase my question about adding in Set\op as, is the common universal Horn theory of Set, Ab, and Set\op, along with their slices, finitely axiomatizable? Or (with or without the slices) is this nice link between Set and Ab confined to the geometric (discrete) half of mathematics? Vaughan