hihi, all - the anti-foundation axiom and some of its consequences are explained in a book by Peter Aczel on non-well-founded sets, though that book is a little too much like the usual (non-naive) set theory for my taste: a good introduction to the axiom is in a book Jon Barwise, John Etchemendy, "The Liar: Essays on Truth and Circularity" i always thought that using sequences instead of sets, the same idea would make an excellent formal theory for the use of pointers and pointer arithmetic, iwht all its attendant dangers and difficulties more later, cal