docs: def remove_coreference(self,coid):
          """
          Removes the coreference with specific identifier
          @type coid: string
          @param coid: the coreference identifier
          """
          for this_node in self.node.findall('coref'):
              if this_node.get('id') == coid:
                  self.node.remove(this_node)
                  break