code: public ImmutableRangeSet<C> union(RangeSet<C> other) {
      return unionOf(Iterables.concat(asRanges(), other.asRanges()));
    }
docs: Returns a new range set consisting of the union of this range set and {@code other}.
  
  <p>This is essentially the same as {@code TreeRangeSet.create(this).addAll(other)} except it
  returns an {@code ImmutableRangeSet}.
  
  @since 21.0