code: public boolean containsAll(Iterable<? extends C> values) {
      if (Iterables.isEmpty(values)) {
        return true;
      }
  
      // this optimizes testing equality of two range-backed sets
      if (values instanceof SortedSet) {
        SortedSet<? extends C> set = cast(values);
        Comparator<?> comparator = set.comparator();
        if (Ordering.natural().equals(comparator) || comparator == null) {
          return contains(set.first()) && contains(set.last());
        }
      }
  
      for (C value : values) {
        if (!contains(value)) {
          return false;
        }
      }
      return true;
    }
docs: Returns {@code true} if every element in {@code values} is {@linkplain #contains contained} in
  this range.