@CanIgnoreReturnValue
  public Caffeine<K, V> ticker(Ticker ticker) {
    requireState(this.ticker == null, "Ticker was already set to %s", this.ticker);
    this.ticker = requireNonNull(ticker);
    return this;
  }
@Test
  public void ticker() {
    Ticker ticker = new FakeTicker()::read;
    var builder = Caffeine.newBuilder().ticker(ticker);
    assertThat(builder.ticker).isSameInstanceAs(ticker);
    assertThat(builder.build()).isNotNull();
  }