code: static <E> ImmutableSortedSet<E> unsafeDelegateSortedSet(
        SortedSet<E> delegate, boolean isSubset) {
      return delegate.isEmpty()
          ? emptySet(delegate.comparator())
          : new RegularImmutableSortedSet<E>(delegate, isSubset);
    }
docs: Assumes that delegate doesn't have null elements and comparator.