code: @SuppressWarnings("unchecked")
    public static <E> TreeMultiset<E> create(@NullableDecl Comparator<? super E> comparator) {
      return (comparator == null)
          ? new TreeMultiset<E>((Comparator) Ordering.natural())
          : new TreeMultiset<E>(comparator);
    }
docs: Creates a new, empty multiset, sorted according to the specified comparator. All elements
  inserted into the multiset must be <i>mutually comparable</i> by the specified comparator:
  {@code comparator.compare(e1, e2)} must not throw a {@code ClassCastException} for any elements
  {@code e1} and {@code e2} in the multiset. If the user attempts to add an element to the
  multiset that violates this constraint, the {@code add(Object)} call will throw a {@code
  ClassCastException}.
  
  @param comparator the comparator that will be used to sort this multiset. A null value
  indicates that the elements' <i>natural ordering</i> should be used.