add lemmas dvd_minus_iff and minus_dvd_iff in class comm_ring_1
## $Id$### targetsdefault: dvi## dependenciesdvi: cd Isar/document; make dvipdf: cd Isar/document; make pdfclean: cd Isar/document; make cleanmrproper: rm -f *.pdf *.dvi cd Isar/document; make mrproper