private boolean lessThan(long ak, long bk, T a, T b) {
      return ak < bk || (ak == bk && lessThanCmp(a, b, tieBreaker));
    }
Merges sorted iterators into a combined iterator over all the items.
planetiler-core/src/main/java/com/onthegomap/planetiler/collection/LongMerger.java