Removes the coreference with specific identifier
        @type coid: string
        @param coid: the coreference identifier

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