code: func (s *scope) errorf(message string, args ...interface{}) error {
  	srcPath, destPath := s.describe()
  	where := fmt.Sprintf("converting %v to %v: ", srcPath, destPath)
  	return fmt.Errorf(where+message, args...)
  }
docs: // error makes an error that includes information about where we were in the objects
  // we were asked to convert.