Goal:
    A : Type  H : ReflectEq A  l2 : list A
  ============================
   (∀ l1 l3 l : list A,
      {| pr1 := []; pr2 := l2 |} = {| pr1 := l1; pr2 := l3 |}
      → ([], [], l2) = split_prefix l1 l3
        → let H := CoreTactics.block in
          is_prefix l l1
          → is_prefix l l3 → is_prefix l (fst (fst (split_prefix l1 l3))))


DEBUG:Going to execute:
simplify * || simplify ?