Returns the previous step before the given `step`. If there are no
        steps available, None will be returned. If the `step` argument is
        None, the current step will be determined automatically.

def get_prev_step(self, step=None):
        """
        Returns the previous step before the given `step`. If there are no
        steps available, None will be returned. If the `step` argument is
        None, the current step will be determined automatically.
        """
        if step is None:
            step = self.steps.current
        form_list = self.get_form_list()
        key = form_list.keyOrder.index(step) - 1
        if key >= 0:
            return form_list.keyOrder[key]
        return None