public static <T extends HasLongSortKey> Iterator<T> mergeSuppliers(List<? extends Supplier<T>> suppliers,
    Comparator<T> tieBreaker) {
    return mergeIterators(suppliers.stream().map(SupplierIterator::new).toList(), tieBreaker);
  }
Merges sorted items from {@link Supplier Suppliers} that return {@code null} when there are no items left.
planetiler-core/src/main/java/com/onthegomap/planetiler/collection/LongMerger.java