-- Mathlib/Computability/PostTuringMachine.lean
theorem tr_respects [Inhabited Γ] :
    Respects (TM1.step M) (TM0.step (tr M))
      fun (c₁ : TM1.Cfg Γ Λ σ) (c₂ : TM0.Cfg Γ (Λ' M)) ↦ trCfg M c₁ = c₂ :=
  fun_respects.2 fun ⟨l₁, v, T⟩ ↦ by
    rcases l₁ with - | l₁; · exact rfl
    simp only [trCfg, TM1.step, FRespects, Option.map]
    induction M l₁ generalizing v T with
    | move _ _ IH => exact TransGen.head rfl (IH _ _)
    | write _ _ IH => exact TransGen.head rfl (IH _ _)
    | load _ _ IH => exact (reaches₁_eq (by rfl)).2 (IH _ _)
    | branch p _ _ IH₁ IH₂ =>
      unfold TM1.stepAux; cases e : p T.1 v
      · exact (reaches₁_eq (by simp only [TM0.step, tr, trAux, e]; rfl)).2 (IH₂ _ _)
      · exact (reaches₁_eq (by simp only [TM0.step, tr, trAux, e]; rfl)).2 (IH₁ _ _)
    | _ =>
      exact TransGen.single (congr_arg some (congr (congr_arg TM0.Cfg.mk rfl) (Tape.write_self T)))

Proof state:
case some.branch.false
Γ : Type u_1
Λ : Type u_2
inst✝² : Inhabited Λ
σ : Type u_3
inst✝¹ : Inhabited σ
M : Λ → TM1.Stmt Γ Λ σ
inst✝ : Inhabited Γ
x✝ : TM1.Cfg Γ Λ σ
l₁ : Λ
p : Γ → σ → Bool
a✝¹ a✝ : TM1.Stmt Γ Λ σ
IH₁ :
  ∀ (v : σ) (T : Tape Γ),
    Reaches₁ (TM0.step (tr M)) { q := (some a✝¹, v), Tape := T }
      {
        q :=
          (match (TM1.stepAux a✝¹ v T).l with
            | some x => some (M x)
            | none => none,
            (TM1.stepAux a✝¹ v T).var),
        Tape := (TM1.stepAux a✝¹ v T).Tape }
IH₂ :
  ∀ (v : σ) (T : Tape Γ),
    Reaches₁ (TM0.step (tr M)) { q := (some a✝, v), Tape := T }
      {
        q :=
          (match (TM1.stepAux a✝ v T).l with
            | some x => some (M x)
            | none => none,
            (TM1.stepAux a✝ v T).var),
        Tape := (TM1.stepAux a✝ v T).Tape }
v : σ
T : Tape Γ
e : p T.head v = false
⊢ Reaches₁ (TM0.step (tr M)) { q := (some (TM1.Stmt.branch p a✝¹ a✝), v), Tape := T }
    {
      q :=
        (match (bif false then TM1.stepAux a✝¹ v T else TM1.stepAux a✝ v T).l with
          | some x => some (M x)
          | none => none,
          (bif false then TM1.stepAux a✝¹ v T else TM1.stepAux a✝ v T).var),
      Tape := (bif false then TM1.stepAux a✝¹ v T else TM1.stepAux a✝ v T).Tape }

Tactic:
exact (reaches₁_eq (by simp only [TM0.step, tr, trAux, e]; rfl)).2 (IH₂ _ _)

Result:
no goals

Proof state:
Γ : Type u_1
Λ : Type u_2
inst✝² : Inhabited Λ
σ : Type u_3
inst✝¹ : Inhabited σ
M : Λ → TM1.Stmt Γ Λ σ
inst✝ : Inhabited Γ
x✝ : TM1.Cfg Γ Λ σ
l₁ : Λ
p : Γ → σ → Bool
a✝¹ a✝ : TM1.Stmt Γ Λ σ
IH₁ :
  ∀ (v : σ) (T : Tape Γ),
    Reaches₁ (TM0.step (tr M)) { q := (some a✝¹, v), Tape := T }
      {
        q :=
          (match (TM1.stepAux a✝¹ v T).l with
            | some x => some (M x)
            | none => none,
            (TM1.stepAux a✝¹ v T).var),
        Tape := (TM1.stepAux a✝¹ v T).Tape }
IH₂ :
  ∀ (v : σ) (T : Tape Γ),
    Reaches₁ (TM0.step (tr M)) { q := (some a✝, v), Tape := T }
      {
        q :=
          (match (TM1.stepAux a✝ v T).l with
            | some x => some (M x)
            | none => none,
            (TM1.stepAux a✝ v T).var),
        Tape := (TM1.stepAux a✝ v T).Tape }
v : σ
T : Tape Γ
e : p T.head v = false
⊢ TM0.step (tr M) { q := (some (TM1.Stmt.branch p a✝¹ a✝), v), Tape := T } =
    TM0.step (tr M) { q := (some a✝, v), Tape := T }

Tactic:
simp only [TM0.step, tr, trAux, e]

Result:
Γ : Type u_1
Λ : Type u_2
inst✝² : Inhabited Λ
σ : Type u_3
inst✝¹ : Inhabited σ
M : Λ → TM1.Stmt Γ Λ σ
inst✝ : Inhabited Γ
x✝ : TM1.Cfg Γ Λ σ
l₁ : Λ
p : Γ → σ → Bool
a✝¹ a✝ : TM1.Stmt Γ Λ σ
IH₁ :
  ∀ (v : σ) (T : Tape Γ),
    Reaches₁ (TM0.step (tr M)) { q := (some a✝¹, v), Tape := T }
      {
        q :=
          (match (TM1.stepAux a✝¹ v T).l with
            | some x => some (M x)
            | none => none,
            (TM1.stepAux a✝¹ v T).var),
        Tape := (TM1.stepAux a✝¹ v T).Tape }
IH₂ :
  ∀ (v : σ) (T : Tape Γ),
    Reaches₁ (TM0.step (tr M)) { q := (some a✝, v), Tape := T }
      {
        q :=
          (match (TM1.stepAux a✝ v T).l with
            | some x => some (M x)
            | none => none,
            (TM1.stepAux a✝ v T).var),
        Tape := (TM1.stepAux a✝ v T).Tape }
v : σ
T : Tape Γ
e : p T.head v = false
⊢ Option.map
      (fun x =>
        { q := x.1,
          Tape :=
            match x.2 with
            | move d => Tape.move d T
            | write a => Tape.write a T })
      (some (bif false then trAux M T.head a✝¹ v else trAux M T.head a✝ v)) =
    Option.map
      (fun x =>
        { q := x.1,
          Tape :=
            match x.2 with
            | move d => Tape.move d T
            | write a => Tape.write a T })
      (some (trAux M T.head a✝ v))