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')
  ============================
   (nth_error (a :: Γ') 0 = Some d'
    → let H := CoreTactics.block in
      let H0 := 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:
simplify * || simplify ?; cbv beta