`UnmodifiableMultimap.forEach` uses the `Multimap.forEach` default implementation which involves listing entries, and creating a new lambda instead of simply passing the BiConsumer through to the protected instance. This is safe to pass directly though because `forEach(BiConsumer)` is a read-only operation.
implements SetMultimap<K, V>, Serializable {
    final Map<K, V> map;
    MapMultimap(Map<K, V> map) {
      this.map = checkNotNull(map);
    }
    @Override
    public int size() {
      return map.size();
    }
    @Override
    public boolean containsKey(Object key) {
      return map.containsKey(key);
    }
    @Override
    public boolean containsValue(Object value) {
      return map.containsValue(value);
    }
    @Override
    public boolean containsEntry(Object key, Object value) {
      return map.entrySet().contains(Maps.immutableEntry(key, value));
    }
    @Override