@Override
      public Integer apply(Integer from) {
        if (++calls > 1) {
          fail();
        }
        throw new MyError();
      }
    };
  }

  // TODO(cpovirk): top-level class?
  static class ExecutorSpy implements Executor {

    Executor delegate;
    boolean wasExecuted;

    public ExecutorSpy(Executor delegate) {
      this.delegate = delegate;
    }

      @Override
      public Integer apply(Integer from) {
        if (++calls > 1) {
          fail();
        }
        throw new MyError();
      }
    };
  }

  // TODO(cpovirk): top-level class?
  static class ExecutorSpy implements Executor {

    Executor delegate;
    boolean wasExecuted;

    public ExecutorSpy(Executor delegate) {
      this.delegate = delegate;
    }