}
        return valueSet;
      }
    }

    @Override
    @CheckForNull
    public V forcePut(@ParametricNullness K key, @ParametricNullness V value) {
      synchronized (mutex) {
        return delegate().forcePut(key, value);
      }
    }

    @Override
    public BiMap<V, K> inverse() {
      synchronized (mutex) {

        }
        return valueSet;
      }
    }

    @Override
    @CheckForNull
    public V forcePut(@ParametricNullness K key, @ParametricNullness V value) {
      synchronized (mutex) {
        return delegate().forcePut(key, value);
      }
    }

    @Override
    public BiMap<V, K> inverse() {
      synchronized (mutex) {

    try {
      unmod.put(4, "four");
      fail("UnsupportedOperationException expected");
    } catch (UnsupportedOperationException expected) {
    }
    try {
      unmod.forcePut(4, "four");
      fail("UnsupportedOperationException expected");
    } catch (UnsupportedOperationException expected) {
    }
    try {
      unmod.putAll(Collections.singletonMap(4, "four"));