code: public static <E extends Comparable<? super E>> ImmutableSortedMultiset<E> of(E element) {
      RegularImmutableSortedSet<E> elementSet =
          (RegularImmutableSortedSet<E>) ImmutableSortedSet.of(element);
      long[] cumulativeCounts = {0, 1};
      return new RegularImmutableSortedMultiset<E>(elementSet, cumulativeCounts, 0, 1);
    }
docs: Returns an immutable sorted multiset containing a single element.