Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  n : nat
  Γ : list A
  a : A
  Γ' : list A
  d' : A
  IHn : ∀ (Γ Γ' : list A) (d' : A),
          All2_fold P Γ Γ'
          → nth_error Γ' n = Some d'
            → ∑ d : A,
                nth_error Γ n = Some d
                × (let Γs := skipn (S n) Γ in
                   let Γs' := skipn (S n) Γ' in
                   All2_fold P Γs Γs' × P Γs Γs' d d')
  Hrel : All2_fold P Γ (a :: Γ')
  ============================
   (let H := CoreTactics.block in
    nth_error (a :: Γ') (S n) = Some d'
    → let H0 := CoreTactics.block in
      ∑ d : A,
        nth_error Γ (S n) = Some d
        × (let Γs := skipn (S (S n)) Γ in
           let Γs' := skipn (S (S n)) (a :: Γ') in
           All2_fold P Γs Γs' × P Γs Γs' d d'))


DEBUG:Going to execute:
fun id cont =>
             let id' := fresh id in
             get_signature_pack id id'; hnf in (value of id');
              hnf in (type of id');
              lazymatch goal with
              | id':=?v:_
                |- context [ id ] =>
                    generalize (eq_refl : v = id'); clearbody id';
                     simpl in id'; cont id id' id v
              | id':=?v:_
                |- _ =>
                    let id'1 := fresh id' in
                    let id'2 := fresh id' in
                    set (id'2 := pr2 id'); set (id'1 := pr1 id') in id'2;
                     hnf in (value of id'1), (value of id'2);
                     try red in (type of id'2);
                     match goal with
                     | id'1:=?t:_
                       |- _ =>
                           generalize (eq_refl : t = id'1); clearbody id'2
                            id'1; clear id' id; try unfold signature in id'2;
                            hnf in id'2; simpl in id'2; rename id'2 into id;
                            cont id id id'1 t
                     end
              end