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

theorem Th23:
  for L being satisfying_DN_1 non empty ComplLLattStr, x being
  Element of L holds x`` = x
proof
  let L be satisfying_DN_1 non empty ComplLLattStr;
  let x be Element of L;
  x`` = (((x + x`)` + x)` + x`)` by Th22
    .= x by Th19;
  hence thesis;
end;
