(STRATEGY
    INNERMOST)

(VAR
    x y)
(DATATYPES
    A = µX.< 0, s(X) >)
(SIGNATURES
    ack :: [A x A] -> A)
(RULES
    ack(0(),y) -> s(y)
    ack(s(x),0()) -> ack(x,s(0()))
    ack(s(x),s(y)) -> ack(x
                         ,ack(s(x),y)))