Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  Γ : list A
  a : A
  Γ' : list A
  Hrel : All2_fold P Γ (a :: Γ')
  Hrel0 := {| pr1 := {| pr1 := Γ; pr2 := a :: Γ' |}; pr2 := Hrel |}
        : sigma
            (λ index : sigma (λ _ : list A, list A),
               All2_fold P (pr1 index) (pr2 index))
  Hrel1 := {| pr1 := Γ; pr2 := a :: Γ' |} : sigma (λ _ : list A, list A)
  Hrel2 := Hrel
        : (λ index : sigma (λ _ : list A, list A),
             All2_fold P (pr1 index) (pr2 index)) Hrel1
  ============================
   (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 a))


DEBUG:Going to execute:
generalize (eq_refl : t = id'1); clearbody id'2 id'1; clear id' id;
            try unfold signature in id'2; hnf in id'2