theorem Th13:
  u1 "\/" u2 = u2 "\/" u1
proof
A1: u1 <= u1"\/"u2 by Lm1;
A2: u2 <= u1"\/"u2 by Lm1;
  for u3 st u2 <= u3 & u1 <= u3 holds u1"\/"u2 <= u3 by Lm1;
  hence thesis by A1,A2,Def13;
end;
