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

theorem Th16:
  for L being satisfying_DN_1 non empty ComplLLattStr, x, y, z
  being Element of L holds ((x + ((x + y)` + z)`)` + z)` = ((x + y)` + z)`
proof
  let L be satisfying_DN_1 non empty ComplLLattStr;
  let x, y, z be Element of L;
  set X = ((x + y)` + x)`, Y = (x + y)`;
  (X + Y)` = x by Th7;
  hence thesis by Th15;
end;
