Goal:
  
  A : Type
  H : ReflectEq A
  l1 : list A
  l2 : list A
  packcall : let H := split_prefix in sigma (λ _ : list A, list A)
  ============================
   (packcall = {| pr1 := l1; pr2 := l2 |}
    → split_prefix (pr1 packcall) (pr2 packcall) =
      split_prefix (pr1 {| pr1 := l1; pr2 := l2 |})
        (pr2 {| pr1 := l1; pr2 := l2 |})
      → let H := CoreTactics.block in
        let
        '(prefix, l1', l2') := split_prefix l1 l2 in
         l1 = prefix ++ l1' ∧ l2 = prefix ++ l2')


DEBUG:Going to execute:
fun tac => match goal with
                      | H:_ |- _ => is_secvar H; tac H
                      end