K k = (K) key;
        return function.apply(k);
      } else {
        return null;
      }
    }

    @Override
    @CheckForNull
    public V remove(@CheckForNull Object key) {
      if (backingSet().remove(key)) {
        @SuppressWarnings("unchecked") // unsafe, but Javadoc warns about it
        K k = (K) key;