public Set<String> getValues() {
        return values;
    }
@Test
    void requireThatValueSetIsMutable() {
        FeatureSet node = new FeatureSet("key");
        node.getValues().add("valueA");
        assertValues(List.of("valueA"), node);

        node = new FeatureSet("key", "valueA");
        node.getValues().add("valueB");
        assertValues(List.of("valueA", "valueB"), node);
    }