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')
  ============================
   (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 =>
             DepElim.generalize_sig id
              ltac:(fun id id' => DepElim.decompose_exists id id')