sclass MetaTransformer {
  sinterface StructureHandler {
    O transform(IF1 f, O o, MetaTransformer mt);
  }
  
  L<structureHandlers> structureHandlers;
  //bool keepUnknown; // assume true for now
  
  O transform(IF1 f, O o) {
    try object f.get(o);
    
    fOr (StructureHandler h : structureHandlers)
      try object h.transform(f, o, this);
    
    //ret keepUnknown ? o : null;
    ret o;
  }
}