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

theorem Th27:
  for L being satisfying_DN_1 non empty ComplLLattStr, x, y being
  Element of L holds x + y = y + x
proof
  let L be satisfying_DN_1 non empty ComplLLattStr;
  let x, y be Element of L;
  x + y = (x + y)`` by Th23
    .= y + x by Th25;
  hence thesis;
end;
