MAYBE

Problem:
 top(free(x)) -> top(check(new(x)))
 new(free(x)) -> free(new(x))
 old(free(x)) -> free(old(x))
 new(serve()) -> free(serve())
 old(serve()) -> free(serve())
 check(free(x)) -> free(check(x))
 check(new(x)) -> new(check(x))
 check(old(x)) -> old(check(x))
 check(old(x)) -> old(x)

Proof:
 Open