Add a child ``<Relationship>`` element with attributes set according
        to parameter values.

def add_rel(self, rId, reltype, target, is_external=False):
        """
        Add a child ``<Relationship>`` element with attributes set according
        to parameter values.
        """
        target_mode = RTM.EXTERNAL if is_external else RTM.INTERNAL
        relationship = CT_Relationship.new(rId, reltype, target, target_mode)
        self.append(relationship)