docs: private void assertSortedByDistance(DoubleDBIDList results) {
      // TODO: sort results instead?
      double dist = -1.0;
      boolean sorted = true;
      for(DoubleDBIDListIter it = results.iter(); it.valid(); it.advance()) {
        double qr = it.doubleValue();
        if(qr < dist) {
          sorted = false;
        }
        dist = qr;
      }
      if(!sorted) {
        try {
          ModifiableDoubleDBIDList.class.cast(results).sort();
        }
        catch(ClassCastException | UnsupportedOperationException e) {
          LoggingUtil.warning("WARNING: results not sorted by distance!", e);
        }
      }
    }