Add the given relation as one or more constraints

        Return a list of the names of the constraints added.

def _add_constraints(self, relation):
        """Add the given relation as one or more constraints

        Return a list of the names of the constraints added.
        """
        expression = relation.expression
        pairs = []
        for value_set in expression.value_sets():
            ind, val = zip(*((self._variables[variable], float(value))
                             for variable, value in value_set))
            pairs.append(cp.SparsePair(ind=ind, val=val))

        names = [next(self._constr_names) for _ in pairs]

        sense = self.CONSTR_SENSE_MAP[relation.sense]
        self._cp.linear_constraints.add(
            names=names, lin_expr=pairs,
            senses=tuple(repeat(sense, len(pairs))),
            rhs=tuple(repeat(float(-expression.offset), len(pairs))))

        return names