Definition write_file (bytes: list memval): file*nat :=
  match write_file_aux bytes (get_size f) fptr with (contents, nwritten) =>
    (mkfile (get_size f + (fptr + nwritten - get_size f)) (get_contents f), nwritten)
  end.Definition fs_read (nbytes: nat): option (list memval) :=
  match get_file fsys fd, get_fptr fsys fd with
  | Some f, Some cur => Some (read_file f cur nbytes)
  | _, _ => None
  end.Definition fs_write (bytes: list memval): option (nat(*nbytes written*)*fs) :=
  match get_file fsys fd, get_fptr fsys fd, get_fmode fsys fd with
  | Some file, Some cur, Some md =>
    if fwritable md then
      match write_file file cur bytes with (new_file, nbytes_written) =>
        Some (nbytes_written,
              mkfs (get_max_fds fsys)
                   (mkfsdata (fun i => if Int.eq fd i then Some (md, cur + nbytes_written)
                                       else get_fdtable fsys i)
                             (fun i => if Int.eq fd i then Some new_file
                                       else get_fcache fsys i)
                             (get_fnames fsys))
                   (get_fstore fsys))
       end
     else None
  | _, _, _ => None
  end.

Lemma fs_write_length bytes n fs: fs_write bytes = Some (n, fs) -> n=length bytes.