theorem Th21:
  X+0.T = X
proof
  thus X+0.T c= X
  proof
    let x be object;
    assume x in X+0.T;
    then ex q being Point of T st x = q + 0.T & q in X;
    hence thesis;
  end;
  let x be object;
  assume
A1: x in X;
  then reconsider x1=x as Point of T;
  x1=x1+0.T;
  hence thesis by A1;
end;
