public FeatureSet addValue(String value) {
        Objects.requireNonNull(value, "value");
        values.add(value);
        return this;
    }
@Test
    void requireThatValueIsMandatoryInSetter() {
        FeatureSet node = new FeatureSet("foo", "bar");
        try {
            node.addValue(null);
            fail();
        } catch (NullPointerException e) {
            assertEquals("value", e.getMessage());
        }
        assertValues(List.of("bar"), node);
    }