code: public static <E, C extends Collection<E>> C combineMutable(C target, C source) {
      target.addAll(source);
      return target;
    }
docs: A combiner for mutable {@link Collection} types.
  Usage: {@code BinaryOperator<List<String>> combiner = CollectionCombiners::combineMutable; }
  
  @param target the instance to call addAll on
  @param source the argument for addAll
  @param <E> the collection element type
  @param <C> the collection type
  @return the target