Goal:
  
  A : Type
  H : ReflectEq A
  l1 : list A
  l2 : list A
  packcall := {| pr1 := l1; pr2 := l2 |} : sigma (λ _ : list A, list A)
  packcall_fn := {|
                   pr1 := packcall;
                   pr2 := split_prefix (pr1 packcall) (pr2 packcall)
                 |}
              : sigma
                  (λ _ : sigma (λ _ : list A, list A),
                     (list A * list A * list A)%type)
  ============================
   (packcall_fn =
    {| pr1 := packcall; pr2 := split_prefix (pr1 packcall) (pr2 packcall) |}
    → 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 packcall =>
             lazymatch goal with
             | |- ?x = ?y → ?P =>
                   let y' := eval unfold packcall in y in
                   change (x = y' → P)
             end