Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  Q : list A → list A → A → A → Type
  d : A
  d' : A
  Γ : list A
  Γ' : list A
  X : All2_fold P Γ Γ'
  p : P Γ Γ' d d'
  IHX : All2_fold Q Γ Γ'
        → All2_fold (λ (Γ Δ : list A) (x y : A), P Γ Δ x y × Q Γ Δ x y) Γ Γ'
  h : All2_fold Q (d :: Γ) (d' :: Γ')
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in
    All2_fold (λ (Γ Δ : list A) (x y : A), P Γ Δ x y × Q Γ Δ x y) 
      (d :: Γ) (d' :: Γ'))


DEBUG:Going to execute:
fun id =>
             DepElim.generalize_sig id
              ltac:(fun id id' => DepElim.decompose_exists id id')