
theorem Th24:
  for L being with_suprema Poset, x,y being Element of L opp holds
  (~x)"\/"(~y) = x"/\"y
proof
  let L be with_suprema Poset, x,y be Element of L opp;
  (~x)~ = ~x & (~y)~ = ~y;
  hence thesis by Th23;
end;
