guava/src/com/google/common/collect/AbstractMapBasedMultimap.java
Collection<V> wrapCollection(@Nullable K key, Collection<V> collection) {
    return new WrappedCollection(key, collection, null);
  }
Collection<V> wrapCollection(@Nullable K key, Collection<V> collection) {
    return new WrappedCollection(key, collection, null);
  }
Generates a decorated collection that remains consistent with the values in the multimap for
the provided key. Changes to the multimap may alter the returned collection, and vice versa.