gcd 0 0 = 0

[Alan Bawden]

   From: Lars Henrik Mathiesen <thorinn@diku.dk>
   Date: 17 Dec 2001 14:50:21 -0000
   ...
   In case it isn't clear already, these definitions make a lattice on
   the positive integers, with divides ~ leq, gcd ~ meet and lcm ~ join,
   using the report's definitions of gcd and lcm.

Indeed, that's a nice way of putting it.  How about if the report just
says:

   In order to make the non-negative integers into a lattice under `gcd'
   and `lcm', we define `gcd 0 0 = 0'.