} else if (delegate.isEmpty()) {
        map.remove(key);
      }
    }

    @ParametricNullness
    K getKey() {
      return key;
    }

    /**
     * Add the delegate to the map. Other {@code WrappedCollection} methods should call this method
     * after adding elements to a previously empty collection.
     *
     * <p>Subcollection add the ancestor's delegate instead.
     */
    void addToMap() {

      } else if (delegate.isEmpty()) {
        map.remove(key);
      }
    }

    @ParametricNullness
    K getKey() {
      return key;
    }

    /**
     * Add the delegate to the map. Other {@code WrappedCollection} methods should call this method
     * after adding elements to a previously empty collection.
     *
     * <p>Subcollection add the ancestor's delegate instead.
     */
    void addToMap() {