reserve n for Nat,
  k for Integer;

theorem Th11:
  (-1)|^2 = 1
proof
  (-1)|^2 = (-1)|^(1+1) .= ((-1)|^1)*((-1)|^1) by NEWTON:8
    .= ((-1)|^1)*(-1)
    .= (-1)*(-1);
  hence thesis;
end;
