2014-07-05 haftmann 2014-07-05 prefer ac_simps collections over separate name bindings for add and mult
2014-07-04 haftmann 2014-07-04 reduced name variants for assoc and commute on plus and mult
2011-09-06 huffman 2011-09-06 avoid using legacy theorem names
2011-01-13 wenzelm 2011-01-13 eliminated global prems; tuned proofs;
2010-08-06 wenzelm 2010-08-06 modernized specifications; tuned headers;
2009-09-01 haftmann 2009-09-01 some reorganization of number theory