def flat(lst, flat_lst = []):
    """делает список одномерным"""
    for lexem in lst:
        if not is_atom_(lexem):
            flat(lexem, flat_lst)
        else:
            flat_lst.append(lexem)
    return flat_lst

def flat2(lst):
    if (len(lst) == 1) and is_atom_(lst[0]):
        return lst
    else:
        return flat2(lst[0:1]) + flat2(lst[1:])

def flat(lst):
    flat_lst = []
    for lexem in lst:
        if not is_atom_(lexem):
            flat_lst.extend(flat(lexem))
        else:
            flat_lst.append(lexem)
    return flat_lst

def flat(lst):
    if is_atom_(lst):
        return [lst]
    return sum(map(flat, lst), [])