'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):

    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
