return false;
    }
    for (Map<C, V> map : backingMap.values()) {
      if (safeContainsKey(map, columnKey)) {
        return true;
      }
    }
    return false;
  }

  @Override
  public boolean containsRow(@Nullable Object rowKey) {
    return rowKey != null && safeContainsKey(backingMap, rowKey);
  }

  @Override
  public boolean containsValue(@Nullable Object value) {

      return new AsMapEntries();
    }

    // The following methods are included for performance.

    @Override
    public boolean containsKey(@Nullable Object key) {
      return Maps.safeContainsKey(submap, key);
    }

    @Override
    public @Nullable Collection<V> get(@Nullable Object key) {
      Collection<V> collection = safeGet(submap, key);
      if (collection == null) {