code: public static <E> Collector<E, ?, ImmutableMultiset<E>> toImmutableMultiset() {
      return toImmutableMultiset(Function.identity(), e -> 1);
    }
docs: Returns a {@code Collector} that accumulates the input elements into a new {@code
  ImmutableMultiset}. Elements iterate in order by the <i>first</i> appearance of that element in
  encounter order.
  
  @since 21.0