* lets all futures share the same listener.
       *
       * We store `localFuturesOrNull` inside the listener because `this.futures` might be nulled
       * out by the time the listener runs for the final future -- at which point we need to check
       * all inputs for exceptions *if* we're collecting values. If we're not, then the listener

        }
      }

      @Override
      public Entry<E> next() {
        if (!hasNext()) {
          throw new NoSuchElementException();
        }
        // requireNonNull is safe because current is only nulled out after iteration is complete.
        Entry<E> result = wrapEntry(requireNonNull(current));
        prevEntry = result;
        if (current.succ() == header) {
          current = null;
        } else {

      if (super.cancel(interruptIfRunning)) {
        /*
         * requireNonNull is generally safe: If cancel succeeded, then this Future was still
         * pending, so its `state` field hasn't been nulled out yet.
         *
         * OK, it's technically possible for this to fail in the presence of unsafe publishing, as
         * discussed in the comments in TimeoutFuture. TODO(cpovirk): Maybe check for null before