MAYBE

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

Strict Trs:
  { ack(0(), y) -> s(y)
  , ack(s(x), y) -> f(x, x)
  , ack(s(x), 0()) -> ack(x, s(0()))
  , ack(s(x), s(y)) -> ack(x, ack(s(x), y))
  , f(x, y) -> ack(x, y)
  , f(x, s(y)) -> f(y, x)
  , f(s(x), y) -> f(x, s(x)) }
Obligation:
  innermost runtime complexity
Answer:
  MAYBE

The input cannot be shown compatible

Arrrr..