Re: Diagrams-briefly

A small side-issue relating to commutative squares concerns introduction of a standard notation for pull-backs and push-outs. In my Pitman book (Research Notes #56) I introduced the following symbols in the middle of the square. For a pull-back, use a small square with upper and left-hand edge removed but top left vertex intact. "Dually" for push-out. TEXperts will know how to encode this. These symbols have found some following among topologists (perhaps by default). No doubt your more ardent correspondents will wish to comment. Jon Berrick. ++++++++++++++++++++++++++++++++++++