'Initiate the initialization.
Checks if the receiving entity advertizes the stream feature. If it
does, the initialization is started. If it is not advertized, and the
C{required} instance variable is C{True}, it raises
L{FeatureNotAdvertized}. Otherwise, the initialization silently
succeeds.'

def initialize(self):

    if (self.feature in self.xmlstream.features):
        return self.start()
    elif self.required:
        raise FeatureNotAdvertized
    else:
        return None
