def _set_step_attrs(self, local_attrs):
        attr_name = '_%ss' % self.name
        if not attr_name in local_attrs:
            local_attrs[attr_name] = []
this method set _givens, _whens and _thens to the class,
dataset/ETHPy150Open hltbra/pyhistorian/pyhistorian/steps.py/Step._set_step_attrs