def _recur_join(s):
    if (len(s) == 0):
        return ''
    elif is_str(s[0]):
        return sep.join(s)
    else:
        s_ = [_recur_join(si) for si in s]
        return _maybe_list_to_array(s_, s)
def _recur_join(s):
    if (len(s) == 0):
        return 
    elif is_str(s[0]):
        return sep.join(s)
    else:
        s_ = [_recur_join(si) for si in s]
        return _maybe_list_to_array(s_, s)