public String getKey() {
        return key;
    }
@Test
    void requireThatConstructorsWork() {
        FeatureSet node = new FeatureSet("key", "valueA", "valueB");
        assertEquals("key", node.getKey());
        assertValues(List.of("valueA", "valueB"), node);

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