
theorem
  for a,b be odd Integer holds min ((a + b) mod 4, (a - b) mod 4) = 0
  proof
    let a,b be odd Integer;
    ((a + b) mod (3 + 1) = 0 or ... or (a + b) mod (3 + 1) = 3) &
    ((a - b) mod (3 + 1) = 0 or ... or (a - b) mod (3 + 1) = 3)
      by NUMBER03:11;
    hence thesis by SDO;
  end;
