code: @Override
    public Collection<V> removeAll(@NullableDecl Object key) {
      Collection<V> collection = map.remove(key);
  
      if (collection == null) {
        return createUnmodifiableEmptyCollection();
      }
  
      Collection<V> output = createCollection();
      output.addAll(collection);
      totalSize -= collection.size();
      collection.clear();
  
      return unmodifiableCollectionSubclass(output);
    }
docs: {@inheritDoc}
  
  <p>The returned collection is immutable.