'Add a relative stem to the current value and return a new RDFURN.
If urn is a fully qualified URN, replace the current value with it.
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):

    if (not isinstance(path, basestring)):
        raise ValueError('Only    strings    should    be    added    to    a    URN.')
    result = self.Copy(age)
    result.Update(path=utils.JoinPath(self._string_urn, path))
    return result
