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

theorem Th22:
  for L being satisfying_DN_1 non empty ComplLLattStr, x, y being
  Element of L holds (((x + y)` + x)` + y)` = y`
proof
  let L be satisfying_DN_1 non empty ComplLLattStr;
  let x, y be Element of L;
  y` = (y + y)` by Th21
    .= (((x + y)` + x)` + y)` by Th17;
  hence thesis;
end;
