code: def Add(self, path, age=None):
      
      if not isinstance(path, string_types):
        raise ValueError("Only strings should be added to a URN.")
  
      result = rdfvalue.RDFURN(self.Copy(age))
      result.Update(path=utils.JoinPath(self._string_urn, path))
  
      return result
docs: Add a relative stem to the current value and return a new RDFURN.
  
      Note that this returns an RDFURN, not a ClientURN since the resulting object
      would not pass validation.
  
      Args:
        path: A string containing a relative path.
        age: The age of the object. If None set to current time.
  
      Returns:
         A new RDFURN that can be chained.
  
      Raises:
         ValueError: if the path component is not a string.