def expression(self, *args, **kwargs):
        e = Expression(*args, _export=False, **kwargs)
        self.model.add_component(e)
        return e
Given a list of nodes, return which group (if any) they represent.
dataset/ETHPy150Open pysb/pysb/pysb/builder.py/Builder.expression--crossover with--dataset/ETHPy150Open andrew-gardner/dependsworkflow/depends/depends_dag.py/DAG.nodeGroupName