extern /*@null@*/ char *
  fgets (/*@returned@*/ /*@out@*/ char *s, int n, FILE *stream)
     /*@modifies fileSystem, *s, *stream, errno@*/
     /*@requires MaxSet(s) >= (n -1); @*/
     /*@ensures MaxRead(s) <= (n -1) /\ MaxRead(s) >= 0; @*/
     ;
