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.

def Add(self, path, age=None):
    """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.
    """
    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