一段時間以來，我一直在嘗試計算語法的后續集，但又遇到了另一個問題。這是我的跟隨集計算器：def gen_follow_set(grammar, start_sym, first_sets):    follow_sets = {nterm: set() for nterm in grammar}    follow_sets[start_sym].add("$")    for _, prods in grammar.items():        for alt in prods:            for item in alt:                if item.isupper():                    follow_sets[item] = set()    while True:        changes = copy.deepcopy(follow_sets)        for nterm, prods in grammar.items():            for alt in prods:                for i, item in enumerate(alt):                    la = alt[i + 1] if i + 1 != len(alt) else nterm                    if i == len(alt) - 1 and item != "":                        follow_sets[item] |= follow_sets[nterm]                    elif item != "":                        if "" in first_sets[la]:                            follow_sets[item] |= first_sets[la].union(                                first_sets[alt[i + 2] if i + 2 <= len(alt) -                                           1 else nterm]) - {""}                        else:                            follow_sets[item] |= first_sets[la]        if changes == follow_sets:            return follow_sets這被稱為：grammar = {    "expr": [["term", "etail"]],    "term": [["LPAREN", "expr", "RPAREN"], ["INT", "ttail"]],    "etail": [["PLUS", "expr"], [""]],    "ttail": [["TIMES", "term"], [""]]}first = calc_first_set(...)pprint.pprint(gen_follow_set(grammar, "expr", first))etail并且expr是正確的，但term并不ttail正確。我怎樣才能得到正確的答案？