theorem Th18:
  k1 "/\" (k1 "\/" k2) = k1
proof
A1: k1 <= k1;
A2: k1 <= k1"\/"k2 by Lm1;
  for k3 st k3 <= k1 & k3 <= k1"\/" k2 holds k3 <= k1;
  hence thesis by A1,A2,Def14;
end;
