theorem Th28:
  |(p+q, p+q)| = |(p, p)| + 2*|(p, q)| + |(q, q)|
proof
  |(p, p)| + |(p, q)| + |(p, q)| = |(p, p)| + 2*|(p, q)|;
  hence thesis by Th26;
end;
