theorem terminates_parallel {S : WSeq (Computation α)} {c} (h : c ∈ S) [T : Terminates c] :
    Terminates (parallel S) := by
  suffices
    ∀ (n) (l : List (Computation α)) (S c),
      c ∈ l ∨ some (some c) = Seq.get? S n → Terminates c → Terminates (corec parallel.aux1 (l, S))
    from
    let ⟨n, h⟩ := h
    this n [] S c (Or.inr h) T
  intro n; induction' n with n IH <;> intro l S c o T
  · cases' o with a a
    · exact terminates_parallel.aux a T
    have H : Seq.destruct S = some (some c, _) := by
      dsimp [Seq.destruct, (· <$> ·)]
      rw [← a]
      simp
      rfl
    induction' h : parallel.aux2 l with a l' <;> have C : corec parallel.aux1 (l, S) = _
    · -- Porting note: To adjust RHS of `C`, these lines are changed.
      apply destruct_eq_pure
      rw [corec_eq, parallel.aux1]
      dsimp only []
      rw [h]
      simp only [rmap]
      rfl
    · rw [C]
      skip
      infer_instance
    · apply destruct_eq_think
      simp only [corec_eq, rmap, parallel.aux1._eq_1]
      rw [h, H]
    · rw [C]
      refine @Computation.think_terminates _ _ ?_
      apply terminates_parallel.aux _ T
      simp
  · cases' o with a a
    · exact terminates_parallel.aux a T
    induction' h : parallel.aux2 l with a l' <;> have C : corec parallel.aux1 (l, S) = _
    · -- Porting note: To adjust RHS of `C`, these lines are changed.
      apply destruct_eq_pure
      rw [corec_eq, parallel.aux1]
      dsimp only []
      rw [h]
      simp only [rmap]
      rfl
    · rw [C]
      skip
      infer_instance
    · apply destruct_eq_think
      simp only [corec_eq, rmap, parallel.aux1._eq_1]
      rw [h]
    · rw [C]
      refine @Computation.think_terminates _ _ ?_
      have TT : ∀ l', Terminates (corec parallel.aux1 (l', S.tail)) := by
        intro
        apply IH _ _ _ (Or.inr _) T
        rw [a]
        cases' S with f al
        rfl
      induction' e : Seq.get? S 0 with o
      · have D : Seq.destruct S = none := by
          dsimp [Seq.destruct]
          rw [e]
          rfl
        rw [D]
        simp only
        have TT := TT l'
        rwa [Seq.destruct_eq_nil D, Seq.tail_nil] at TT
      · have D : Seq.destruct S = some (o, S.tail) := by
          dsimp [Seq.destruct]
          

Proof state:
α : Type u
β : Type v
S✝ : WSeq (Computation α)
c✝ : Computation α
h✝ : c✝ ∈ S✝
T✝ : Terminates c✝
n : ℕ
IH :
  ∀ (l : List (Computation α)) (S : Stream'.Seq (Option (Computation α))) (c : Computation α),
    c ∈ l ∨ some (some c) = Seq.get? S n → Terminates c → Terminates (corec parallel.aux1 (l, S))
l : List (Computation α)
S : Stream'.Seq (Option (Computation α))
c : Computation α
T : Terminates c
a : some (some c) = Seq.get? S (Nat.succ n)
l' : List (Computation α)
h : parallel.aux2 l = Sum.inr l'
C :
  corec parallel.aux1 (l, S) =
    think
      (corec parallel.aux1
        (match Seq.destruct S with
        | none => (l', Seq.nil)
        | some (none, S') => (l', S')
        | some (some c, S') => (c :: l', S')))
TT : ∀ (l' : List (Computation α)), Terminates (corec parallel.aux1 (l', Seq.tail S))
o : Option (Computation α)
e : Seq.get? S 0 = some o
⊢ Option.map (fun a' => (a', Seq.tail S)) (Seq.get? S 0) = some (o, Seq.tail S)

Next tactic:
rw [e]