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

theorem Th11:
  for L being satisfying_DN_1 non empty ComplLLattStr, x, y, z
  being Element of L holds ((((x + y)` + z)` + (x` + y)`)` + y)` = (x` + y)`
proof
  let L be satisfying_DN_1 non empty ComplLLattStr;
  let x, y, z be Element of L;
  set X = (x + y)`, Z = (x` + y)`, Y = z;
  (((X + Y)` + Z)` + (X + Z)`)` = Z by Th9;
  hence thesis by Th6;
end;
