def unindex(self, participation):
        """Remove an existing participation from the index
        """
        #if the Participation is not linked to an Avatar there's no point to
        #   unindex it
        a = participation.getAvatar()

        if not a:
            return
        #if the Avatar associated to the participation isn't in the index do
        #   nothing
        if not self._idx.has_key(a.getId()):
            return
        #if the given participation is indexed remove it, otherwise do nothing
        if participation in self._idx[a.getId()]:
            self._idx[a.getId()].remove(participation)