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'
  ============================
   (let H := CoreTactics.block in
    ∑ 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:
tac H
Level 4: evaluation returns
idtac of type tacvalue