Goal:
  
  A : Type
  B : Type
  C : Type
  Q : C → Type
  R : C → A → Prop
  f : ∀ x : A, (∀ y : C, Q y → R y x) → B
  packcall : let H := map_All f in
             sigma (λ l : list A, ∀ y : C, Q y → ∥ All (R y) l ∥)
  ============================
   (∀ (P : C → A → Prop) (Q' : B → Type) (args : list A) 
      (ha : ∀ y : C, Q y → ∥ All (R y) args ∥),
      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:
match goal with
           | |- ?T => change (let _ := CoreTactics.block in T)
           end