reserve n for Nat,
  k for Integer;

theorem Th12:
  for n being Nat holds (-1)|^n = (-1)|^(n+2)
proof
  let n be Nat;
  (-1)|^(n+2) = ((-1)|^n)*((-1)|^2) by NEWTON:8
    .= (-1)|^n by Th11;
  hence thesis;
end;
