Adds an coreference to the coreference layer
        @type coreference: L{Ccoreference}
        @param coreference: the coreference object

def add_coreference(self, coreference):
        """
        Adds an coreference to the coreference layer
        @type coreference: L{Ccoreference}
        @param coreference: the coreference object
        """
        if self.coreference_layer is None:
            self.coreference_layer = Ccoreferences(type=self.type)
            self.root.append(self.coreference_layer.get_node())
        self.coreference_layer.add_coreference(coreference)