Isabelle 如何引用Isar中的当前子目标？

我试图从中解决练习4.7。 我遇到了一个案件，在这个案件中，我证明了我的错误，因此证明了一切，但我无法结案，因为我不知道如何引用我的证明义务

theory ProgProveEx47
  imports Main
begin

datatype alpha = a | b | c

inductive S :: "alpha list ⇒ bool" where
  Nil: "S []" |
  Grow: "S xs ⟹ S ([a]@xs@[b])" |
  Append: "S xs ⟹ S ys ⟹ S (xs@ys)"

fun balanced :: "nat ⇒ alpha list ⇒ bool" where
  "balanced 0 [] = True" |
  "balanced (Suc n) (b#xs) = balanced n xs" |
  "balanced n (a#xs) = balanced (Suc n) xs" |
  "balanced _ _ = False"

lemma
  fixes n xs
  assumes b: "balanced n xs"
  shows "S (replicate n a @ xs)"
proof -
  from b show ?thesis
  proof (induction xs)
    case Nil
    hence "S (replicate n a)"
    proof (induction n)
      case 0
      show ?case using S.Nil by simp
      case (Suc n)
      value ?case
      from `balanced (Suc n) []` have False by simp
      (* thus "S (replicate (Suc n) a)" by simp *)
      (* thus ?case by simp *)
      then show "⋀n. (balanced n [] ⟹ S (replicate n a)) ⟹ balanced (Suc n) [] ⟹ S (replicate (Suc n) a)" by simp

上次

演出后的命题是从Isabelle/jedit中的证明状态复制而来的。但是，Isabelle报告了错误（在上次的
show
）：

被注释掉的证明目标现在导致了同样的错误。如果我将案例交换为
0
和
Suc
，则
0
案例的最后一次
show
会出现错误，但
Suc
案例不再出现错误

有人能解释为什么伊莎贝尔不会接受这些看似正确的目标吗？我怎样才能以伊莎贝尔会接受的方式陈述子目标？是否有一个通用的方法来引用当前的子目标？我认为，考虑到我使用的结构，
？case
应该可以完成这项工作，但显然不行

我发现了堆栈溢出问题，其中提到了相同的错误，但问题不同（定理有一个等价性，应该通过隐式应用
规则
，将其分解为定向子目标）在我的例子中，应用提供的解决方案会导致不正确和无法实现的目标。
在内部归纳证明中，您只是缺少了一个
下一个

lemma
  fixes n xs
  assumes b: "balanced n xs"
  shows "S (replicate n a @ xs)"
proof -
  from b show ?thesis
  proof (induction xs)
    case Nil
    hence "S (replicate n a)"
    proof (induction n)
      case 0
      show ?case using S.Nil by simp
    next (* this next was missing *)
      case (Suc n)
      show ?case sorry
    qed
    show ?case sorry
  next
    case (Cons a xs)
    then show ?case sorry
  qed

lemma
  fixes n xs
  assumes b: "balanced n xs"
  shows "S (replicate n a @ xs)"
proof -
  from b show ?thesis
  proof (induction xs)
    case Nil
    hence "S (replicate n a)"
    proof (induction n)
      case 0
      show ?case using S.Nil by simp
    next (* this next was missing *)
      case (Suc n)
      show ?case sorry
    qed
    show ?case sorry
  next
    case (Cons a xs)
    then show ?case sorry
  qed