BiMap<Integer, String> mod = HashBiMap.create();
    mod.put(1, "one");
    mod.put(2, "two");
    mod.put(3, "three");

    BiMap<Number, String> unmod = Maps.<Number, String>unmodifiableBiMap(mod);

    /* No aliasing on inverse operations. */
    assertThat(unmod.inverse()).isSameInstanceAs(unmod.inverse());
    assertThat(unmod.inverse().inverse()).isSameInstanceAs(unmod);