
theorem MRS:
  for a,b be non zero Nat holds (a - b) mod 2*b = (a + b) mod 2*b
  proof
    let a,b be non zero Nat;
    ((a - b) + 2*b) mod (2*b) =
       (((a - b) mod (2*b)) + ((2*b) mod (2*b))) mod (2*b) by NAT_D:66;
    hence thesis;
  end;
