code: public void merge(UnsafeExternalSorter other) throws IOException {
      other.spill();
      spillWriters.addAll(other.spillWriters);
      // remove them from `spillWriters`, or the files will be deleted in `cleanupResources`.
      other.spillWriters.clear();
      other.cleanupResources();
    }
docs: Merges another UnsafeExternalSorters into this one, the other one will be emptied.
  
  @throws IOException