docs: 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)