Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  Γ : list A
  Γ' : list A
  a : A
  Δ : list A
  a0 : A
  Δ' : list A
  H : #|Γ| = #|Γ'|
  IHΔ : All2_fold P (Δ ++ Γ) (Δ' ++ Γ')
        → All2_fold P Γ Γ'
          × All2_fold (λ Δ Δ' : list A, P (Δ ++ Γ) (Δ' ++ Γ')) Δ Δ'
  H'1 : sigma (λ _ : list A, list A)
  H' : All2_fold P (pr1 H'1) (pr2 H'1)
  ============================
   ({| pr1 := a :: Δ ++ Γ; pr2 := a0 :: Δ' ++ Γ' |} = H'1
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in
      All2_fold P Γ Γ'
      × All2_fold (λ Δ Δ' : list A, P (Δ ++ Γ) (Δ' ++ Γ')) 
          (a :: Δ) (a0 :: Δ'))


DEBUG:Going to execute:
try rename id into id',id' into id
Level 3: evaluation returns
try rename id into id',id' into id; cont id'1 id
where
t := {| pr1 := a :: Δ ++ Γ; pr2 := a0 :: Δ' ++ Γ' |}
of type
constr
id'1 := H'1
of type
constr
id' := H'
of type
constr
id := H'
of type
constr
cont := fun id id' => DepElim.decompose_exists id id'
        where
        id := H'
        of type
        constr
of type
tacvalue of type tacvalue