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

theorem Th28:
  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;
  ((x + y)` + x)` + y = y`` by Th24;
  hence thesis by Th23;
end;
