docs: private void findMatches(ModifiableDBIDs posn, Relation<?> lrelation, Object label) {
      posn.clear();
      for(DBIDIter ri = lrelation.iterDBIDs(); ri.valid(); ri.advance()) {
        if(match(label, lrelation.get(ri))) {
          posn.add(ri);
        }
      }
    }