code: @Override
    public void addAll(Map<K, V> m) {
      for (Map.Entry<K, V> e : m.entrySet()) {
        add(e.getKey(), e.getValue());
      }
    }
docs: Adds all of the mappings in m to this CollectionValuedMap.
  If m is a CollectionValuedMap, it will behave strangely. Use the constructor instead.