code: func (t *Transport) RegisterLocation(loc string, path string) {
  	t.mu.Lock()
  	defer t.mu.Unlock()
  	if t.loc == nil {
  		t.loc = make(map[string]string)
  	}
  	if _, exists := t.loc[loc]; exists {
  		panic("location " + loc + " already registered")
  	}
  	t.loc[loc] = path
  }
docs: // RegisterLocation registers an URL location and maps it to the given
  // file system path.
  //
  // Calling RegisterLocation twice for the same location is a
  // programmer error, and causes a panic.