
theorem Th22:
  for L being with_infima Poset, x,y being Element of L opp holds
  (~x)"/\"(~y) = x"\/"y
proof
  let L be with_infima Poset, x,y be Element of L opp;
  (~x)~ = ~x & (~y)~ = ~y;
  hence thesis by Th21;
end;
