def add_external_reference_to_role(self,role_id,ext_ref):
        """
        Adds an external reference to a role identifier
        @param role_id: the role identifier
        @type role_id: string
        @param ext_ref: the external reference
        @type ext_ref: L{CexternalReference}
        """
        node_role = self.map_roleid_node[role_id]
        obj_role = Crole(node_role)
        obj_role.add_external_reference(ext_ref)
Adds an external reference to a role identifier
        @param role_id: the role identifier
        @type role_id: string
        @param ext_ref: the external reference
        @type ext_ref: L{CexternalReference}