Goal:
  
  A : Type
  B : Type
  C : Type
  Q : C → Type
  P : C → A → Prop
  Q' : B → Type
  R : C → A → Prop
  f : ∀ x : A, (∀ y : C, Q y → R y x) → B
  args : list A
  ha : ∀ y : C, Q y → ∥ All (R y) args ∥
  packcall := {| pr1 := args; pr2 := ha |}
           : sigma (λ l : list A, ∀ y : C, Q y → ∥ All (R y) l ∥)
  packcall_fn := {|
                   pr1 := packcall;
                   pr2 := map_All f (pr1 packcall) (pr2 packcall)
                 |}
              : sigma
                  (λ _ : sigma (λ l : list A, ∀ y : C, Q y → ∥ All (R y) l ∥),
                     list B)
  ============================
   (packcall = {| pr1 := args; pr2 := ha |}
    → map_All f (pr1 packcall) (pr2 packcall) =
      map_All f (pr1 {| pr1 := args; pr2 := ha |})
        (pr2 {| pr1 := args; pr2 := ha |})
      → let H := CoreTactics.block in
        (∀ y : C, Q y → All (P y) args)
        → (∀ (x : A) (y : C) (rx : ∀ y0 : C, Q y0 → R y0 x),
             P y x → Q' (f x rx)) → ∀ y : C, Q y → All Q' (map_All f args ha))


DEBUG:Going to execute:
intros H Heq; try (rewrite <- Heq; clear Heq);
            CoreTactics.revert_until H; revert H; subst packcall_fn;
            clearbody packcall; make_packcall packcall elimfn;
            with_last_secvar ltac:(fun eos => move packcall before eos)
             ltac:(move packcall at top); CoreTactics.revert_until packcall;
            CoreTactics.block_goal; cbv zeta in packcall