anchor.diff:    VS operator [](Object key) => _multimap[key];
   
     @override
  -  void operator []=(K key, VS value) =>
  -      _multimap.replaceAll(key, value);
  +  void operator []=(K key, VS value) => _multimap.replaceAll(key, value);
   
     @override
     void clear() => _multimap.clear();
  
positive.code: }
  
  // Internal helper to populate a [multimap] from an [Iterable] of entries.
  void fillFromEntries<K, V, VS extends Iterable<V>>(Multimap<K, V, VS> multimap,
      Iterable<MapEntry<K, V>> entries) {
    for (final entry in entries) {
      multimap.add(entry.key, entry.value);
    }
  
negative.code:   void remove(K key, V value) => lookupValues(key).polymorphicRemove(value);
  
    /// Removes associations from [key] to each of [values].
    void removeAll(K key, [Iterable<V> values]) => values == null
        ? lookupValues(key).polymorphicClear()
        : lookupValues(key).polymorphicRemoveAll(values);
  
    /// Replaces the [key] with a single [value].
    void replace(K key, V value) => lookupValues(key)
      ..polymorphicClear()
      ..polymorphicAdd(value);
  
    /// Replaces the [key] with each of the [values].
    void replaceAll(K key, Iterable<V> values) => lookupValues(key)
      ..polymorphicClear()
      ..polymorphicAddAll(values);
  
    /// Removes all values from this multimap.
    void clear() {
  