code: public final <C extends Collection<? super T>> C copyInto(C collection) {
          requireNonNull(collection);
          for (T item : this) {
              collection.add(item);
          }
          return collection;
      }
docs: Copies all the elements from this fluent iterable to {@code collection}. This is equivalent to
  calling {@code Iterables.addAll(collection, this)}.