Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  Γ : list A
  a : A
  Γ' : list A
  Hrel1' : list A
  Hrel1 : list A
  Hrel : All2_fold P Hrel1' Hrel1
  ============================
   ({| pr1 := Γ; pr2 := a :: Γ' |} = {| pr1 := Hrel1'; pr2 := 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:
fun id id' =>
             hnf in id;
              match type of id with
              | sigma _ =>
                  let xn := fresh id "'" in
                  destruct id as [xn id]; DepElim.decompose_exists xn id;
                   cbv beta iota delta [TODO projection TODO projection]
                    in id, id'; DepElim.decompose_exists id id'
              | _ =>
                  cbv beta iota delta [TODO projection TODO projection]
                   in id, id'
              end