typedef struct File <Record> {
  int mode; /* rw .. */
  Key *key;
  Ack *ack;
  Record *record;
  Data *fileImpl ;

  __code open(FileImpl* file,Key *key, int mode, __code next(Record *record,...));
  __code ack(FileImpl* file,Ack *ack, __code next(Record *record,...));
} File;