source: class HistoryTest {
  
    private static final TestKey ABLE;
    private static final TestKey BAKER;
    private static final TestKey CHARLIE;
  
    @Test public void builderPopToExplodesOnMissingState() {
  
target:     History.Builder builder = History.emptyBuilder();
      builder.push(ABLE);
      builder.push(BAKER);
      builder.push(CHARLIE);
      try {
        builder.popTo(new Object());
        fail("Missing state object, should have thrown");
      } catch (IllegalArgumentException ignored) {
        // Correct!
      }
    }
  
  }