Returns the inverse view of this bimap, which maps each of this bimap's
   * values to its associated key. The two bimaps are backed by the same data;
   * any changes to one will appear in the other.
Returns the inverse view of this bimap, which maps each of this bimap's
   * values to its associated key. The two bimaps are backed by the same data;
   * any changes to one will appear in the other. | BiMap.java