record UndoCreateConcept(long conceptID) > Undo {
    toString { ret "Create " + getConcept(conceptID); }
    
    S undo() {
      Concept c = getConcept(conceptID);
      S result = "Deleted " + c + " again";
      deleteConcept(conceptID);
      ret result;
    }
  }

  record UndoDeleteConcept(Concept concept) > Undo {
    toString { ret "Delete " + concept; }
    
    S undo() {
      registerConcept(concept);
      ret "Restored " + concept;
    }
  }

  record UndoSetConceptField(long conceptID, S field, O value, O newValue) > Undo {
    toString { ret "Set " + field + " of " + getConcept(conceptID) + " to " + newValue; }
    
    S undo() {
      Concept c = getConcept(conceptID);
      S result = "Reset " + field + " of " + c + " to " + value;
      cset(c, field, value);
      ret result;
    }
  }