/**
   * Returns a set of row keys that have one or more values in the table. Changes to the set will
   * update the underlying table, and vice versa.
   *
   * @return set of row keys
   */
  Set<R> rowKeySet();

  /**
   * Returns a set of column keys that have one or more values in the table. Changes to the set will
   * update the underlying table, and vice versa.
   *
   * @return set of column keys
   */

      synchronized (mutex) {
        return set(delegate().cellSet(), mutex);
      }
    }

    @Override
    public Set<R> rowKeySet() {
      synchronized (mutex) {
        return set(delegate().rowKeySet(), mutex);
      }
    }

    @Override
    public Set<C> columnKeySet() {
      synchronized (mutex) {
        return set(delegate().columnKeySet(), mutex);

      synchronized (mutex) {
        return set(delegate().cellSet(), mutex);
      }
    }

    @Override
    public Set<R> rowKeySet() {
      synchronized (mutex) {
        return set(delegate().rowKeySet(), mutex);
      }
    }

    @Override
    public Set<C> columnKeySet() {
      synchronized (mutex) {
        return set(delegate().columnKeySet(), mutex);