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


DEBUG:Going to execute:
intro H