    @Test
    public void keys() {

        final var given = ArrayListMultimap.create();

        given.put("key1", "item1");
        given.put("key1", "item1");
        given.put("key2", "item2");

        final var values = List.of("key1", "key1", "key2");
        final var expected = new ArrayListMultiset<>(values);

        final var actual = given.keys();

        assertEquals(expected, actual);
    }