Goal:
  
  A : Type
  H : ReflectEq A
  packcall : let H := split_prefix in sigma (λ _ : list A, list A)
  ============================
   (∀ l1 l2 : 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:
change (let _ := CoreTactics.block in T)