'Populate the stanza object using an optional XML object.
Overrides ElementBase.setup
Caches identity and feature information.
Arguments:
xml -- Use an existing XML object for the stanza\'s values.'

def setup(self, xml=None):

    ElementBase.setup(self, xml)
    self._identities = set([id[0:3] for id in self['identities']])
    self._features = self['features']
