}
        return valueSet;
      }
    }

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

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

    /* UnsupportedOperationException on direct modifications. */
    assertThrows(UnsupportedOperationException.class, () -> unmod.put(4, "four"));
    assertThrows(UnsupportedOperationException.class, () -> unmod.forcePut(4, "four"));
    assertThrows(UnsupportedOperationException.class, () -> unmod.putAll(singletonMap(4, "four")));

    /* UnsupportedOperationException on indirect modifications. */