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


DEBUG:Going to execute:
fun tac H =>
             DepElim.do_intros H; DepElim.generalize_by_eqs H; tac H