typedef struct UnixFileImpl <> ipml File {
  int fd;    // unix file descriptor
  int mode; 
  Data *fileImpl ;
  __code unixOpen(UnixFileImpl* file,Key *key, __code next(Block *block,...));
  __code uniAck(UnixFileImpl* file,Ack *ack, __code next(Block *block,...));
} File;