@staticmethod
def get_previous_symbol(curr_future_symbol: str) ->str:
    """
        >>> assert(EminiFuture.get_previous_symbol('ESH9') == 'ESZ8')
        """
    month = curr_future_symbol[2]
    year = int(curr_future_symbol[3])
    prev_month = {'H': 'Z', 'M': 'H', 'U': 'M', 'Z': 'U'}[month]
    prev_year = year if prev_month != 'Z' else year - 1
    if prev_year == -1:
        prev_year == 9
    return f'ES{prev_month}{prev_year}'
