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_fn =
    {| pr1 := packcall; pr2 := map_All f (pr1 packcall) (pr2 packcall) |}
    → 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:
lazymatch goal with
           | |- ?x = ?y → ?P =>
                 let y' := eval unfold packcall in y in
                 change (x = y' → P)
           end