sclass MetaTransformer {
  sinterface StructureHandler {
    O transform(IF1 f, O o, MetaTransformer mt);
  }
  
  L<StructureHandler> structureHandlers;
  //bool keepUnknown; // assume true for now
  
  *() {}
  *(StructureHandler... handlers) { structureHandlers = asList(handlers); }
  
  // if f returns null, go through structure
  O transform(IF1 f, O o) {
    ping();
    try object f.get(o);
    
    fOr (StructureHandler h : structureHandlers) {
      ping();
      try object h.transform(f, o, this);
    }
    
    //ret keepUnknown ? o : null;
    ret o;
  }
  
  IF1 asFunction(IF1 f) {
    ret o -> transform(f, o);
  }
}