code: def get_prev_step(self, step=None):
          
          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
docs: 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.