Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  d : A
  Γ0 : list A
  a : A
  Γ' : list A
  Hrel : All2_fold P Γ0 Γ'
  p : P Γ0 Γ' d a
  ============================
   (∑ d0 : A,
      nth_error (d :: Γ0) 0 = Some d0
      × (let Γs := skipn 1 (d :: Γ0) in
         let Γs' := skipn 1 (a :: Γ') in All2_fold P Γs Γs' × P Γs Γs' d0 a))


DEBUG:Going to execute:
unfold CoreTactics.block in *; cbv zeta