long instant = 0L;
    final List<String> events = Lists.newArrayList();

    @Override
    public long readMicros() {
      return NANOSECONDS.toMicros(instant);
    }

    void sleepMillis(int millis) {
      sleepMicros("U", MILLISECONDS.toMicros(millis));
    }

    void sleepMicros(String caption, long micros) {
      instant += MICROSECONDS.toNanos(micros);

         */
        return new Iterator<T>() {
          boolean atStart = true;

          @Override
          public boolean hasNext() {
            return iterator.hasNext();
          }

          @Override
          @ParametricNullness
          public T next() {
            T result = iterator.next();
            atStart = false; // not called if next() fails
            return result;
          }

 * successive readings of "now" in the same process.
 *
 * <p>In contrast, <i>wall time</i> is a reading of "now" as given by a method like
 * {@link System#currentTimeMillis()}, best represented as an {@link java.time.Instant}. Such values
 * <i>can</i> be subtracted to obtain a {@code Duration} (such as by {@code Duration.between}), but
 * doing so does <i>not</i> give a reliable measurement of elapsed time, because wall time readings

    long instant = 0L;
    final List<String> events = Lists.newArrayList();

    @Override
    public long readMicros() {
      return NANOSECONDS.toMicros(instant);
    }

    void sleepMillis(int millis) {
      sleepMicros("U", MILLISECONDS.toMicros(millis));
    }

    void sleepMicros(String caption, long micros) {
      instant += MICROSECONDS.toNanos(micros);

   * of input futures.
   *
   * <p>See {@link #whenAllComplete} and {@link #whenAllSucceed} for how to instantiate this class.
   *
   * <p>Example:
   *
   * <pre>{@code
   * final ListenableFuture<Instant> loginDateFuture =
   *     loginService.findLastLoginDate(username);
   * final ListenableFuture<List<String>> recentCommandsFuture =
   *     recentCommandsService.findRecentCommands(username);

 * successive readings of "now" in the same process.
 *
 * <p>In contrast, <i>wall time</i> is a reading of "now" as given by a method like
 * {@link System#currentTimeMillis()}, best represented as an {@link java.time.Instant}. Such values
 * <i>can</i> be subtracted to obtain a {@code Duration} (such as by {@code Duration.between}), but
 * doing so does <i>not</i> give a reliable measurement of elapsed time, because wall time readings

         */
        return new Iterator<T>() {
          boolean atStart = true;

          @Override
          public boolean hasNext() {
            return iterator.hasNext();
          }

          @Override
          @ParametricNullness
          public T next() {
            T result = iterator.next();
            atStart = false; // not called if next() fails
            return result;
          }