theorem Th15:
  n1 "/\" n2 = n2 "/\" n1
proof
A1: n1"/\"n2 <= n1 by Lm2;
A2: n1"/\"n2 <= n2 by Lm2;
  for n3 st n3 <= n2 & n3 <= n1 holds n3 <= n1"/\"n2 by Lm2;
  hence thesis by A1,A2,Def14;
end;
