reserve L for satisfying_DN_1 non empty ComplLLattStr;
reserve x, y, z for Element of L;

theorem Th19:
  for L being satisfying_DN_1 non empty ComplLLattStr, x, y being
  Element of L holds ((x + y)` + y`)` = y
proof
  let L be satisfying_DN_1 non empty ComplLLattStr;
  let x, y be Element of L;
  (y` + (x + y)`)` = y by Th18;
  hence thesis by Th14;
end;
