theorem Th17:
  (k1 "/\" k2) "\/" k2 = k2
proof
A1: k1"/\"k2 <= k2 by Lm2;
A2: k2 <= k2;
  for k3 st k1"/\" k2 <= k3 & k2 <= k3 holds k2 <= k3;
  hence thesis by A1,A2,Def13;
end;
