sclass ConceptsRefChecker {
  Concepts cc;
  new L<Err> errors;
  
  sclass Err {}
  srecord ErrNoBackRef(Concept.Ref ref, Concept dest) > Err {}
  srecord ErrSuperfluousBackRef(Concept.Ref ref, Concept dest) > Err {}
  
  // a reference from a concept that is not in the concepts list
  srecord ErrDanglingSource(Concept.Ref ref, Concept dest) > Err {}
  
  // a reference to a concept that is not in the concepts list
  srecord ErrDanglingDestination(Concept.Ref ref, Concept dest) > Err {}
  
  *() {}
  *(Concepts *cc) {}
  
  L<Err> run() {
    errors.clear();
    if (cc == null) ret errors;
    for (Concept c : cc.allConcepts()) {
      for (Concept.Ref ref : c.backRefs) {
        if (ref! != c)
          errors.add(new ErrSuperfluousBackRef(ref, c));
        if (ref.concept()._concepts != cc)
          errors.add(new ErrDanglingSource(ref, c));
      }
          
      for (Concept.Ref ref : c.refs)
        if (ref.has()) {
          if (!contains(ref->backRefs, ref))
            errors.add(new ErrNoBackRef(ref, c));
          if (ref->_concepts != cc)
            errors.add(new ErrDanglingDestination(ref, c));
      }
    }
    ret errors;
  }
  
  L<Err> errors() { ret errors; }
}