// type parameter <S> lets us avoid the extra <String> in statements like:
  // Ordering<String> o = Ordering.<String>natural().nullsFirst();
  public <S extends T> Ordering<@Nullable S> nullsFirst() {
    return new NullsFirstOrdering<S>(this);
  }

  /**
   * Returns an ordering that treats {@code null} as greater than all other values and uses this
   * ordering to compare non-null values.
   *