* natural ordering. The sorted multisets use {@link Ordering#natural()} as the comparator. This
   * method provides more type-safety than {@link #builder}, as it can be called only for classes
   * that implement {@link Comparable}.
   *
   * <p>Note: the type parameter {@code E} extends {@code Comparable<?>} rather than {@code
   * Comparable<? super E>} as a workaround for javac <a