typedef struct file <Impl> {
    union Data* file;
    int          ref;   // reference count
    char         readable;
    char         writable;
    unsigned int off;
    struct stat* st;
    char* addr;
    int n;
    __code stat(Impl* file, struct stat* st, __code next(...));
    __code read(Impl* file, char* addr, __code next(...));
    __code write(Impl* file, char* addr, int n, __code next(...));
    __code close(Impl* file,__code next(...));
} file;