-- Module: Mathlib.Data.WSeq.Basic
Goal:
α : Type u
l : List α
s : WSeq α
s1 s2 : Computation (List α)
h :
  ∃ (l' : List α) (s : WSeq α),
    s1 =
        Computation.corec
          (fun (x : List α × WSeq α) ↦
            match (motive := List α × WSeq α → List α ⊕ List α × WSeq α) x with
            | (l, s) =>
              match (motive := Option (Seq1 (Option α)) → List α ⊕ List α × WSeq α) Seq.destruct s with
              | none => Sum.inl l.reverse
              | some (none, s') => Sum.inr (l, s')
              | some (some a, s') => Sum.inr (a :: l, s'))
          (l' ++ l, s) ∧
      s2 =
        Computation.map (fun (x : List α) ↦ l.reverse ++ x)
          (Computation.corec
            (fun (x : List α × WSeq α) ↦
              match (motive := List α × WSeq α → List α ⊕ List α × WSeq α) x with
              | (l, s) =>
                match (motive := Option (Seq1 (Option α)) → List α ⊕ List α × WSeq α) Seq.destruct s with
                | none => Sum.inl l.reverse
                | some (none, s') => Sum.inr (l, s')
                | some (some a, s') => Sum.inr (a :: l, s'))
            (l', s))
rcases :
  ∀ (l' : List α) (s : WSeq α)
    (h :
      s1 =
          Computation.corec
            (fun (x : List α × WSeq α) ↦
              match (motive := List α × WSeq α → List α ⊕ List α × WSeq α) x with
              | (l, s) =>
                match (motive := Option (Seq1 (Option α)) → List α ⊕ List α × WSeq α) Seq.destruct s with
                | none => Sum.inl l.reverse
                | some (none, s') => Sum.inr (l, s')
                | some (some a, s') => Sum.inr (a :: l, s'))
            (l' ++ l, s) ∧
        s2 =
          Computation.map (fun (x : List α) ↦ l.reverse ++ x)
            (Computation.corec
              (fun (x : List α × WSeq α) ↦
                match (motive := List α × WSeq α → List α ⊕ List α × WSeq α) x with
                | (l, s) =>
                  match (motive := Option (Seq1 (Option α)) → List α ⊕ List α × WSeq α) Seq.destruct s with
                  | none => Sum.inl l.reverse
                  | some (none, s') => Sum.inr (l, s')
                  | some (some a, s') => Sum.inr (a :: l, s'))
              (l', s))),
    BisimO
      (fun (c1 c2 : Computation (List α)) ↦
        ∃ (l' : List α) (s : WSeq α),
          c1 =
              Computation.corec
                (fun (x : List α × WSeq α) ↦
                  match (motive := List α × WSeq α → List α ⊕ List α × WSeq α) x with
                  | (l, s) =>
                    match (motive := Option (Seq1 (Option α)) → List α ⊕ List α × WSeq α) Seq.destruct s with
                    | none => Sum.inl l.reverse
                    | some (none, s') => Sum.inr (l, s')
                    | some (some a, s') => Sum.inr (a :: l, s'))
                (l' ++ l, s) ∧
            c2 =
              Computation.map (fun (x : List α) ↦ l.reverse ++ x)
                (Computation.corec
                  (fun (x : List α × WSeq α) ↦
                    match (motive := List α × WSeq α → List α ⊕ List α × WSeq α) x with
                    | (l, s) =>
                      match (motive := Option (Seq1 (Option α)) → List α ⊕ List α × WSeq α) Seq.destruct s with
                      | none => Sum.inl l.reverse
                      | some (none, s') => Sum.inr (l, s')
                      | some (some a, s') => Sum.inr (a :: l, s'))
                  (l', s)))
      s1.destruct s2.destruct
⊢ BisimO
    (fun (c1 c2 : Computation (List α)) ↦
      ∃ (l' : List α) (s : WSeq α),
        c1 =
            Computation.corec
              (fun (x : List α × WSeq α) ↦
                match (motive := List α × WSeq α → List α ⊕ List α × WSeq α) x with
                | (l, s) =>
                  match (motive := Option (Seq1 (Option α)) → List α ⊕ List α × WSeq α) Seq.destruct s with
                  | none => Sum.inl l.reverse
                  | some (none, s') => Sum.inr (l, s')
                  | some (some a, s') => Sum.inr (a :: l, s'))
              (l' ++ l, s) ∧
          c2 =
            Computation.map (fun (x : List α) ↦ l.reverse ++ x)
              (Computation.corec
                (fun (x : List α × WSeq α) ↦
                  match (motive := List α × WSeq α → List α ⊕ List α × WSeq α) x with
                  | (l, s) =>
                    match (motive := Option (Seq1 (Option α)) → List α ⊕ List α × WSeq α) Seq.destruct s with
                    | none => Sum.inl l.reverse
                    | some (none, s') => Sum.inr (l, s')
                    | some (some a, s') => Sum.inr (a :: l, s'))
                (l', s)))
    s1.destruct s2.destruct

Tactic:
rcases h with ⟨l', s, h⟩