Goal:
  
  A : Type
  B : Type
  C : Type
  Q : C → Type
  P : C → A → Prop
  fn : ∀ x : A, (∀ y : C, Q y → P y x) → B
  l : list A
  H : ∀ y : C, Q y → ∥ All (P y) l ∥
  n : nat
  x : A
  H0 : nth_error l n = Some x
  packcall : let H := map_All fn in
             sigma (λ l : list A, ∀ y : C, Q y → ∥ All (P y) l ∥)
  ============================
   (packcall = {| pr1 := l; pr2 := H |}
    → map_All fn (pr1 packcall) (pr2 packcall) =
      map_All fn (pr1 {| pr1 := l; pr2 := H |})
        (pr2 {| pr1 := l; pr2 := H |})
      → let H0 := CoreTactics.block in
        ∃ D : ∀ y : C, Q y → P y x,
          nth_error (map_All fn l H) n = Some (fn x D))


DEBUG:Going to execute:
with_last_secvar_aux tac
Level 2: evaluation returns
with_last_secvar_aux tac + orelse
where
tac := fun eos => move packcall before eos
       where
       tac := fun _ => idtac
              where
              c := map_All fn l H
              of type
              constr
       of type
       tacvalue
       packcall_fn := packcall_fn
       of type
       intropattern
       packcall := packcall
       of type
       intropattern
       elimn := 3
       of type
       constr
       elimfn := map_All fn
       of type
       constr
       elimc := fun_elim
       of type
       constr
       c := map_All fn l H
       of type
       constr
       Heq := Heqcall
       of type
       intropattern
       H := eqargs
       of type
       intropattern
of type
tacvalue
orelse := move packcall at top
          where
          tac := fun _ => idtac
                 where
                 c := map_All fn l H
                 of type
                 constr
          of type
          tacvalue
          packcall_fn := packcall_fn
          of type
          intropattern
          packcall := packcall
          of type
          intropattern
          elimn := 3
          of type
          constr
          elimfn := map_All fn
          of type
          constr
          elimc := fun_elim
          of type
          constr
          c := map_All fn l H
          of type
          constr
          Heq := Heqcall
          of type
          intropattern
          H := eqargs
          of type
          intropattern
of type
tacvalue of type tacvalue