-- Mathlib/Data/Seq/Parallel.lean
theorem exists_of_mem_parallel {S : WSeq (Computation α)} {a} (h : a ∈ parallel S) :
    ∃ c ∈ S, a ∈ c

Proof state:
case cons.inr.inr.intro.intro.inl
α : Type u
S : WSeq (Computation α)
a : α
h✝ : a ∈ parallel S
F : List (Computation α) → α ⊕ List (Computation α) → Prop :=
  fun l a =>
    Sum.casesOn a (fun a => ∃ c, c ∈ l ∧ a ∈ c) fun l' => ∀ (a' : α), (∃ c, c ∈ l' ∧ a' ∈ c) → ∃ c, c ∈ l ∧ a' ∈ c
c : Computation α
l val✝ : List (Computation α)
IH : F l (Sum.inr val✝)
c' : Computation α
h : c.destruct = Sum.inr c'
a' : α
d : Computation α
ad : a' ∈ c'
e : d = c'
⊢ a' ∈ c'.think

Tactic:
exact think_mem ad

Result:
no goals

Proof state:
case cons.inl
α : Type u
S : WSeq (Computation α)
a : α
h : a ∈ parallel S
F : List (Computation α) → α ⊕ List (Computation α) → Prop :=
  fun l a =>
    Sum.casesOn a (fun a => ∃ c, c ∈ l ∧ a ∈ c) fun l' => ∀ (a' : α), (∃ c, c ∈ l' ∧ a' ∈ c) → ∃ c, c ∈ l ∧ a' ∈ c
c : Computation α
l : List (Computation α)
val✝ : α
IH : F l (Sum.inl val✝)
⊢ F (c :: l) (Sum.inl val✝)

Tactic:
rcases IH with ⟨c', cl, ac⟩

Result:
case cons.inl.intro.intro
α : Type u
S : WSeq (Computation α)
a : α
h : a ∈ parallel S
F : List (Computation α) → α ⊕ List (Computation α) → Prop :=
  fun l a =>
    Sum.casesOn a (fun a => ∃ c, c ∈ l ∧ a ∈ c) fun l' => ∀ (a' : α), (∃ c, c ∈ l' ∧ a' ∈ c) → ∃ c, c ∈ l ∧ a' ∈ c
c : Computation α
l : List (Computation α)
val✝ : α
c' : Computation α
cl : c' ∈ l
ac : val✝ ∈ c'
⊢ F (c :: l) (Sum.inl val✝)

Proof state:
case cons.inr.inr.intro.intro.inl
α : Type u
S : WSeq (Computation α)
a : α
h✝ : a ∈ parallel S
F : List (Computation α) → α ⊕ List (Computation α) → Prop :=
  fun l a =>
    Sum.casesOn a (fun a => ∃ c, c ∈ l ∧ a ∈ c) fun l' => ∀ (a' : α), (∃ c, c ∈ l' ∧ a' ∈ c) → ∃ c, c ∈ l ∧ a' ∈ c
c : Computation α
l val✝ : List (Computation α)
IH : F l (Sum.inr val✝)
c' : Computation α
h : c.destruct = Sum.inr c'
a' : α
d : Computation α
ad : a' ∈ c'
e : d = c'
⊢ ∃ c_1, c_1 ∈ c :: l ∧ a' ∈ c_1

Tactic:
refine ⟨c, List.mem_cons_self _ _, ?_⟩

Result:
case cons.inr.inr.intro.intro.inl
α : Type u
S : WSeq (Computation α)
a : α
h✝ : a ∈ parallel S
F : List (Computation α) → α ⊕ List (Computation α) → Prop :=
  fun l a =>
    Sum.casesOn a (fun a => ∃ c, c ∈ l ∧ a ∈ c) fun l' => ∀ (a' : α), (∃ c, c ∈ l' ∧ a' ∈ c) → ∃ c, c ∈ l ∧ a' ∈ c
c : Computation α
l val✝ : List (Computation α)
IH : F l (Sum.inr val✝)
c' : Computation α
h : c.destruct = Sum.inr c'
a' : α
d : Computation α
ad : a' ∈ c'
e : d = c'
⊢ a' ∈ c

Proof state:
case h2.inr.some.mk
α : Type u
S✝ : WSeq (Computation α)
a : α
h : a ∈ parallel S✝
F : List (Computation α) → α ⊕ List (Computation α) → Prop :=
  fun l a =>
    Sum.casesOn a (fun a => ∃ c, c ∈ l ∧ a ∈ c) fun l' => ∀ (a' : α), (∃ c, c ∈ l' ∧ a' ∈ c) → ∃ c, c ∈ l ∧ a' ∈ c
lem1 : ∀ (l : List (Computation α)), F l (parallel.aux2 l)
C : Computation α
aC : a ∈ C
C' : Computation α
IH :
  ∀ (l : List (Computation α)) (S : WSeq (Computation α)),
    corec parallel.aux1 (l, S) = C' → ∃ c, (c ∈ l ∨ c ∈ S) ∧ a ∈ c
l : List (Computation α)
S : WSeq (Computation α)
e✝ : corec parallel.aux1 (l, S) = C'.think
l' : List (Computation α)
this : F l (Sum.inr l')
o : Option (Computation α)
S' : Stream'.Seq (Option (Computation α))
h' :
  corec parallel.aux1
      ((fun l' =>
          match some (o, S') with
          | none => (l', Seq.nil)
          | some (none, S') => (l', S')
          | some (some c, S') => (c :: l', S'))
        l') =
    C'
e : Seq.destruct S = some (o, S')
⊢ ∃ c, (c ∈ l ∨ c ∈ S) ∧ a ∈ c

Tactic:
obtain - | c := o <;> simp [parallel.aux1] at h' <;> rcases IH _ _ h' with ⟨d, dl | dS', ad⟩

Result:
case h2.inr.some.mk.none.intro.intro.inl
α : Type u
S✝ : WSeq (Computation α)
a : α
h : a ∈ parallel S✝
F : List (Computation α) → α ⊕ List (Computation α) → Prop :=
  fun l a =>
    Sum.casesOn a (fun a => ∃ c, c ∈ l ∧ a ∈ c) fun l' => ∀ (a' : α), (∃ c, c ∈ l' ∧ a' ∈ c) → ∃ c, c ∈ l ∧ a' ∈ c
lem1 : ∀ (l : List (Computation α)), F l (parallel.aux2 l)
C : Computation α
aC : a ∈ C
C' : Computation α
IH :
  ∀ (l : List (Computation α)) (S : WSeq (Computation α)),
    corec parallel.aux1 (l, S) = C' → ∃ c, (c ∈ l ∨ c ∈ S) ∧ a ∈ c
l : List (Computation α)
S : WSeq (Computation α)
e✝ : corec parallel.aux1 (l, S) = C'.think
l' : List (Computation α)
this : F l (Sum.inr l')
S' : Stream'.Seq (Option (Computation α))
h' : corec parallel.aux1 (l', S') = C'
e : Seq.destruct S = some (none, S')
d : Computation α
ad : a ∈ d
dl : d ∈ l'
⊢ ∃ c, (c ∈ l ∨ c ∈ S) ∧ a ∈ c

case h2.inr.some.mk.none.intro.intro.inr
α : Type u
S✝ : WSeq (Computation α)
a : α
h : a ∈ parallel S✝
F : List (Computation α) → α ⊕ List (Computation α) → Prop :=
  fun l a =>
    Sum.casesOn a (fun a => ∃ c, c ∈ l ∧ a ∈ c) fun l' => ∀ (a' : α), (∃ c, c ∈ l' ∧ a' ∈ c) → ∃ c, c ∈ l ∧ a' ∈ c
lem1 : ∀ (l : List (Computation α)), F l (parallel.aux2 l)
C : Computation α
aC : a ∈ C
C' : Computation α
IH :
  ∀ (l : List (Computation α)) (S : WSeq (Computation α)),
    corec parallel.aux1 (l, S) = C' → ∃ c, (c ∈ l ∨ c ∈ S) ∧ a ∈ c
l : List (Computation α)
S : WSeq (Computation α)
e✝ : corec parallel.aux1 (l, S) = C'.think
l' : List (Computation α)
this : F l (Sum.inr l')
S' : Stream'.Seq (Option (Computation α))
h' : corec parallel.aux1 (l', S') = C'
e : Seq.destruct S = some (none, S')
d : Computation α
ad : a ∈ d
dS' : d ∈ S'
⊢ ∃ c, (c ∈ l ∨ c ∈ S) ∧ a ∈ c

case h2.inr.some.mk.some.intro.intro.inl
α : Type u
S✝ : WSeq (Computation α)
a : α
h : a ∈ parallel S✝
F : List (Computation α) → α ⊕ List (Computation α) → Prop :=
  fun l a =>
    Sum.casesOn a (fun a => ∃ c, c ∈ l ∧ a ∈ c) fun l' => ∀ (a' : α), (∃ c, c ∈ l' ∧ a' ∈ c) → ∃ c, c ∈ l ∧ a' ∈ c
lem1 : ∀ (l : List (Computation α)), F l (parallel.aux2 l)
C : Computation α
aC : a ∈ C
C' : Computation α
IH :
  ∀ (l : List (Computation α)) (S : WSeq (Computation α)),
    corec parallel.aux1 (l, S) = C' → ∃ c, (c ∈ l ∨ c ∈ S) ∧ a ∈ c
l : List (Computation α)
S : WSeq (Computation α)
e✝ : corec parallel.aux1 (l, S) = C'.think
l' : List (Computation α)
this : F l (Sum.inr l')
S' : Stream'.Seq (Option (Computation α))
c : Computation α
h' : corec parallel.aux1 (c :: l', S') = C'
e : Seq.destruct S = some (some c, S')
d : Computation α
ad : a ∈ d
dl : d ∈ c :: l'
⊢ ∃ c, (c ∈ l ∨ c ∈ S) ∧ a ∈ c

case h2.inr.some.mk.some.intro.intro.inr
α : Type u
S✝ : WSeq (Computation α)
a : α
h : a ∈ parallel S✝
F : List (Computation α) → α ⊕ List (Computation α) → Prop :=
  fun l a =>
    Sum.casesOn a (fun a => ∃ c, c ∈ l ∧ a ∈ c) fun l' => ∀ (a' : α), (∃ c, c ∈ l' ∧ a' ∈ c) → ∃ c, c ∈ l ∧ a' ∈ c
lem1 : ∀ (l : List (Computation α)), F l (parallel.aux2 l)
C : Computation α
aC : a ∈ C
C' : Computation α
IH :
  ∀ (l : List (Computation α)) (S : WSeq (Computation α)),
    corec parallel.aux1 (l, S) = C' → ∃ c, (c ∈ l ∨ c ∈ S) ∧ a ∈ c
l : List (Computation α)
S : WSeq (Computation α)
e✝ : corec parallel.aux1 (l, S) = C'.think
l' : List (Computation α)
this : F l (Sum.inr l')
S' : Stream'.Seq (Option (Computation α))
c : Computation α
h' : corec parallel.aux1 (c :: l', S') = C'
e : Seq.destruct S = some (some c, S')
d : Computation α
ad : a ∈ d
dS' : d ∈ S'
⊢ ∃ c, (c ∈ l ∨ c ∈ S) ∧ a ∈ c