Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  Γ : list A
  a : A
  Γ' : list A
  d' : A
  Hrel : All2_fold P Γ (a :: Γ')
  H : nth_error (a :: Γ') 0 = Some d'
  ============================
   (∑ d : A,
      nth_error Γ 0 = Some d
      × (let Γs := skipn 1 Γ in
         let Γs' := skipn 1 (a :: Γ') in All2_fold P Γs Γs' × P Γs Γs' d d'))


DEBUG:Going to execute:
change (let _ := CoreTactics.block in T)
DEBUG LTAC Evaluated term: (let H := CoreTactics.block in
                            ∑ d : A,
                              nth_error Γ 0 = Some d
                              × (let Γs := skipn 1 Γ in
                                 let Γs' := skipn 1 [] in
                                 All2_fold P Γs Γs' × P Γs Γs' d d'))