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'
  Hrel0 := {| pr1 := {| pr1 := a :: Γ; pr2 := Γ' |}; pr2 := Hrel |}
        : sigma
            (λ index : sigma (λ _ : list A, list A),
               All2_fold P (pr1 index) (pr2 index))
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in
    ∑ 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:
let id'1 := fresh id' in
           let id'2 := fresh id' in
           set (id'2 := pr2 id'); set (id'1 := pr1 id') in id'2;
            hnf in (value of id'1), (value of id'2);
            try red in (type of id'2);
            match goal with
            | id'1:=?t:_
              |- _ =>
                  generalize (eq_refl : t = id'1); clearbody id'2 id'1; clear
                   id' id; try unfold signature in id'2; 
                   hnf in id'2; simpl in id'2; rename id'2 into id;
                   cont id id id'1 t
            end