-- Mathlib/Data/Seq/Parallel.lean
theorem terminates_parallel.aux :
    ∀ {l : List (Computation α)} {S c},
      c ∈ l → Terminates c → Terminates (corec parallel.aux1 (l, S))

Proof state:
case cons.inr
α : Type u
lem1 :
  ∀ (l : List (Computation α)) (S : WSeq (Computation α)),
    (∃ a, parallel.aux2 l = Sum.inl a) → (corec parallel.aux1 (l, S)).Terminates
c✝ : Computation α
T : c✝.Terminates
s : Computation α
IH : ∀ {l : List (Computation α)} {S : WSeq (Computation α)}, s ∈ l → (corec parallel.aux1 (l, S)).Terminates
S : WSeq (Computation α)
c : Computation α
l : List (Computation α)
IH' : s.think ∈ l → ∀ (l' : List (Computation α)), parallel.aux2 l = Sum.inr l' → s ∈ l'
l' : List (Computation α)
e' :
  (match
      List.foldr
        (fun c o =>
          match o with
          | Sum.inl a => Sum.inl a
          | Sum.inr ls =>
            match c.destruct with
            | Sum.inl a => Sum.inl a
            | Sum.inr b => Sum.inr (b :: ls))
        (Sum.inr []) l with
    | Sum.inl a => Sum.inl a
    | Sum.inr ls =>
      match c.destruct with
      | Sum.inl a => Sum.inl a
      | Sum.inr b => Sum.inr (b :: ls)) =
    Sum.inr l'
m : s.think ∈ l
⊢ s ∈ l'

Tactic:
induction' e : List.foldr (fun c o =>
    match o with
    | Sum.inl a => Sum.inl a
    | Sum.inr ls => rmap (fun c' => c' :: ls) (destruct c))
  (Sum.inr List.nil) l with a' ls <;> erw [e] at e'

Result:
case cons.inr.inl
α : Type u
lem1 :
  ∀ (l : List (Computation α)) (S : WSeq (Computation α)),
    (∃ a, parallel.aux2 l = Sum.inl a) → (corec parallel.aux1 (l, S)).Terminates
c✝ : Computation α
T : c✝.Terminates
s : Computation α
IH : ∀ {l : List (Computation α)} {S : WSeq (Computation α)}, s ∈ l → (corec parallel.aux1 (l, S)).Terminates
S : WSeq (Computation α)
c : Computation α
l : List (Computation α)
IH' : s.think ∈ l → ∀ (l' : List (Computation α)), parallel.aux2 l = Sum.inr l' → s ∈ l'
l' : List (Computation α)
m : s.think ∈ l
a' : α
e' :
  (match Sum.inl a' with
    | Sum.inl a => Sum.inl a
    | Sum.inr ls =>
      match c.destruct with
      | Sum.inl a => Sum.inl a
      | Sum.inr b => Sum.inr (b :: ls)) =
    Sum.inr l'
e :
  List.foldr
      (fun c o =>
        match o with
        | Sum.inl a => Sum.inl a
        | Sum.inr ls => rmap (fun c' => c' :: ls) c.destruct)
      (Sum.inr []) l =
    Sum.inl a'
⊢ s ∈ l'

case cons.inr.inr
α : Type u
lem1 :
  ∀ (l : List (Computation α)) (S : WSeq (Computation α)),
    (∃ a, parallel.aux2 l = Sum.inl a) → (corec parallel.aux1 (l, S)).Terminates
c✝ : Computation α
T : c✝.Terminates
s : Computation α
IH : ∀ {l : List (Computation α)} {S : WSeq (Computation α)}, s ∈ l → (corec parallel.aux1 (l, S)).Terminates
S : WSeq (Computation α)
c : Computation α
l : List (Computation α)
IH' : s.think ∈ l → ∀ (l' : List (Computation α)), parallel.aux2 l = Sum.inr l' → s ∈ l'
l' : List (Computation α)
m : s.think ∈ l
ls : List (Computation α)
e' :
  (match Sum.inr ls with
    | Sum.inl a => Sum.inl a
    | Sum.inr ls =>
      match c.destruct with
      | Sum.inl a => Sum.inl a
      | Sum.inr b => Sum.inr (b :: ls)) =
    Sum.inr l'
e :
  List.foldr
      (fun c o =>
        match o with
        | Sum.inl a => Sum.inl a
        | Sum.inr ls => rmap (fun c' => c' :: ls) c.destruct)
      (Sum.inr []) l =
    Sum.inr ls
⊢ s ∈ l'

Proof state:
case a
α : Type u
l : List (Computation α)
S : WSeq (Computation α)
a : α
e : parallel.aux2 l = Sum.inl a
⊢ (match
      match parallel.aux2 l with
      | Sum.inl a => Sum.inl a
      | Sum.inr b =>
        Sum.inr
          (match Seq.destruct S with
          | none => (b, Seq.nil)
          | some (none, S') => (b, S')
          | some (some c, S') => (c :: b, S')) with
    | Sum.inl a => Sum.inl a
    | Sum.inr b => Sum.inr (corec parallel.aux1 b)) =
    Sum.inl a

Tactic:
rw [e]

Result:
no goals

Proof state:
α : Type u
lem1 :
  ∀ (l : List (Computation α)) (S : WSeq (Computation α)),
    (∃ a, parallel.aux2 l = Sum.inl a) → (corec parallel.aux1 (l, S)).Terminates
c : Computation α
T : c.Terminates
s : Computation α
IH : ∀ {l : List (Computation α)} {S : WSeq (Computation α)}, s ∈ l → (corec parallel.aux1 (l, S)).Terminates
l : List (Computation α)
S : WSeq (Computation α)
m : s.think ∈ l
⊢ ∀ (l' : List (Computation α)), parallel.aux2 l = Sum.inr l' → s ∈ l'

Tactic:
induction' l with c l IH' <;> intro l' e' <;> simp at m

Result:
case cons
α : Type u
lem1 :
  ∀ (l : List (Computation α)) (S : WSeq (Computation α)),
    (∃ a, parallel.aux2 l = Sum.inl a) → (corec parallel.aux1 (l, S)).Terminates
c✝ : Computation α
T : c✝.Terminates
s : Computation α
IH : ∀ {l : List (Computation α)} {S : WSeq (Computation α)}, s ∈ l → (corec parallel.aux1 (l, S)).Terminates
S : WSeq (Computation α)
c : Computation α
l : List (Computation α)
IH' : s.think ∈ l → ∀ (l' : List (Computation α)), parallel.aux2 l = Sum.inr l' → s ∈ l'
l' : List (Computation α)
e' : parallel.aux2 (c :: l) = Sum.inr l'
m : s.think = c ∨ s.think ∈ l
⊢ s ∈ l'

Proof state:
case cons.inl.h_1
α : Type u
lem1 :
  ∀ (l : List (Computation α)) (S : WSeq (Computation α)),
    (∃ a, parallel.aux2 l = Sum.inl a) → (corec parallel.aux1 (l, S)).Terminates
c✝ : Computation α
T : c✝.Terminates
s : Computation α
IH : ∀ {l : List (Computation α)} {S : WSeq (Computation α)}, s ∈ l → (corec parallel.aux1 (l, S)).Terminates
S : WSeq (Computation α)
c : Computation α
l : List (Computation α)
IH' : s.think ∈ l → ∀ (l' : List (Computation α)), parallel.aux2 l = Sum.inr l' → s ∈ l'
l' : List (Computation α)
e : s.think = c
o✝ : α ⊕ List (Computation α)
a✝ : α
heq✝ :
  List.foldr
      (fun c o =>
        match o with
        | Sum.inl a => Sum.inl a
        | Sum.inr ls =>
          match c.destruct with
          | Sum.inl a => Sum.inl a
          | Sum.inr b => Sum.inr (b :: ls))
      (Sum.inr []) l =
    Sum.inl a✝
⊢ Sum.inl a✝ = Sum.inr l' → s ∈ l'

Tactic:
simp

Result:
no goals

Proof state:
case a.cons.inr
α : Type u
lem1 :
  ∀ (l : List (Computation α)) (S : WSeq (Computation α)),
    (∃ a, parallel.aux2 l = Sum.inl a) → (corec parallel.aux1 (l, S)).Terminates
c✝ : Computation α
T : c✝.Terminates
a : α
S : WSeq (Computation α)
c : Computation α
l : List (Computation α)
IH : pure a ∈ l → ∃ a, parallel.aux2 l = Sum.inl a
m : pure a ∈ l
⊢ ∃ a, parallel.aux2 (c :: l) = Sum.inl a

Tactic:
obtain ⟨a', e⟩ := IH m

Result:
case a.cons.inr.intro
α : Type u
lem1 :
  ∀ (l : List (Computation α)) (S : WSeq (Computation α)),
    (∃ a, parallel.aux2 l = Sum.inl a) → (corec parallel.aux1 (l, S)).Terminates
c✝ : Computation α
T : c✝.Terminates
a : α
S : WSeq (Computation α)
c : Computation α
l : List (Computation α)
IH : pure a ∈ l → ∃ a, parallel.aux2 l = Sum.inl a
m : pure a ∈ l
a' : α
e : parallel.aux2 l = Sum.inl a'
⊢ ∃ a, parallel.aux2 (c :: l) = Sum.inl a