code: public void addAll(Map<K, V> m) {
      if (m instanceof CollectionValuedMap<?, ?>) {
        throw new UnsupportedOperationException();
      }
      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.