*
   * <p><b>Performance note:</b> the instance returned is a singleton.
   */
  @SuppressWarnings("unchecked")
  public static <R, C, V> ImmutableTable<R, C, V> of() {
    return (ImmutableTable<R, C, V>) SparseImmutableTable.EMPTY;
  }

  /** Returns an immutable table containing a single cell. */
  public static <R, C, V> ImmutableTable<R, C, V> of(R rowKey, C columnKey, V value) {