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

theorem Th7:
  for L being satisfying_DN_1 non empty ComplLLattStr, x, y being
  Element of L holds (((x + y)` + x)` + (x + y)`)` = x
proof
  let L be satisfying_DN_1 non empty ComplLLattStr;
  let x, y be Element of L;
  set X = (x + y)`;
  ((X + x)` + ((x + X)` + (((x + x`)` + x)` + (x + y)`)`)`)` = x by Th4;
  hence thesis by Th5;
end;
