
theorem OpIn01:
  for a being Element of [.0,1.] holds
    1 - a in [.0,1.]
  proof
    let a be Element of [.0,1.];
    0 <= a <= 1 by XXREAL_1:1; then
    1 - 1 <= 1 - a <= 1 - 0 by XREAL_1:13;
    hence thesis by XXREAL_1:1;
  end;
