Goal:
    A : Type  H : ReflectEq A  a : A  l : list A
  ============================
   (∀ l1 l2 : list A,
      {| pr1 := a :: l; pr2 := [] |} = {| pr1 := l1; pr2 := l2 |}
      → ([], a :: l, []) = split_prefix l1 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:
intro H