code: func (g *Graph) String(def ...string) string {
  
  	// If g is nil, return default or nothing
  	if g == nil {
  		if len(def) == 0 {
  			return ""
  		}
  		return def[0]
  	}
  
  	return _string(g.Interface())
  }
docs: // String returns a string representation of this node, or an empty string.
  // This function doesn't return an error, because it is mostly used in single
  // variable return situations.
  // String accepts one default value, which will be returned instead of an
  // empty string.