docs: 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)