// to be sorted based on the top of each iterator, with insertion order as tiebreaker
      Comparator<IndexedIterator<T>> heapComparator =
          (o1, o2) ->
              ComparisonChain.start()
                  .compare(o1.iterator.peek(), o2.iterator.peek(), itemComparator)
                  // When elements are equal, use insertion order to maintain stability