Goal:
    A : Type  H : ReflectEq A  a : A  l : list A
  ============================
   (∀ l1 l2 l0 : list A,
      {| pr1 := a :: l; pr2 := [] |} = {| pr1 := l1; pr2 := l2 |}
      → ([], a :: l, []) = split_prefix l1 l2
        → let H := CoreTactics.block in
          is_prefix l0 l1
          → is_prefix l0 l2 → is_prefix l0 (fst (fst (split_prefix l1 l2))))


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