'Given a volume URN return the relative URN as a unicode string.
We remove the volume prefix from our own.
Args:
volume: An RDFURN or fully qualified url string.
Returns:
A string of the url relative from the volume or None if our URN does not
start with the volume prefix.'

def RelativeName(self, volume):

    string_url = utils.SmartUnicode(self)
    volume_url = utils.SmartUnicode(volume)
    if string_url.startswith(volume_url):
        result = string_url[len(volume_url):]
        return result.lstrip('/')
    return None
