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


DEBUG:Going to execute:
unfold CoreTactics.block in *