float path_len(town * src, town * dst, int dep = 0)
{ src->been = true;
  cout << setw(3 * dep) << "" << "At " << src->name << "\n";
  if (src == dst)
  { src->been = false;
    return 0.0; }
  for (road * r: src->exits)
  { town * there = other_end(r, src);
    if (there->been)
      continue;
    float len = path_len(there, dst, dep + 1);
    if (len >= 0.0)
    { src->been = false;
      return len + r->length; } }
  src->been = false;
  return -1.0; }

...

int main(int argc, char * argv[])
{ if (argc < 3 || argc > 4)
  { cerr << "correct usage: " << argv[0] << " start destination [mapfile]\n";
    exit(1); }
  string filename = "/home/www/class/een318/nebraska-gr.txt";      
  if (argc == 4)
    filename = argv[3];
  read_map(filename);                                
  ...