@Nullable protected V get(K key) {
    if (key == null) return null;
    Object[] state = state();
    int i = indexOfExistingKey(state, key);
    return i != -1 ? (V) state[i + 1] : null;
  }
@Test void get_ignored_if_unconfigured() {
    assertThat(extra.get("three")).isNull();
  }