MAYBE

We are left with following problem, upon which TcT provides the
certificate MAYBE.

Strict Trs:
  { isEmpty(empty()) -> true()
  , isEmpty(node(l, x, r)) -> false()
  , left(empty()) -> empty()
  , left(node(l, x, r)) -> l
  , right(empty()) -> empty()
  , right(node(l, x, r)) -> r
  , elem(node(l, x, r)) -> x
  , append(nil(), x) -> cons(x, nil())
  , append(cons(y(), ys), x) -> cons(y(), append(ys, x))
  , listify(n, xs) ->
    if(isEmpty(n),
       isEmpty(left(n)),
       right(n),
       node(left(left(n)),
            elem(left(n)),
            node(right(left(n)), elem(n), right(n))),
       xs,
       append(xs, n))
  , if(true(), b, n, m, xs, ys) -> xs
  , if(false(), true(), n, m, xs, ys) -> listify(n, ys)
  , if(false(), false(), n, m, xs, ys) -> listify(m, xs)
  , toList(n) -> listify(n, nil()) }
Obligation:
  innermost runtime complexity
Answer:
  MAYBE

The input cannot be shown compatible

Arrrr..