public void testTreeMultimapDerived() {
    TreeMultimap<DerivedComparable, DerivedComparable> multimap = TreeMultimap.create();
    assertEquals(ImmutableMultimap.of(), multimap);
    multimap.put(new DerivedComparable("foo"), new DerivedComparable("f"));
    multimap.put(new DerivedComparable("foo"), new DerivedComparable("o"));
    multimap.put(new DerivedComparable("foo"), new DerivedComparable("o"));