TIMEOUT
MAYBE
Trs:
 {  top(free(x)) -> top(check(new(x))),
    new(serve()) -> free(serve()),
    new(free(x)) -> free(new(x)),
  check(free(x)) -> free(check(x)),
   check(new(x)) -> new(check(x)),
   check(old(x)) -> old(check(x)),
   check(old(x)) -> old(x),
    old(serve()) -> free(serve()),
    old(free(x)) -> free(old(x))}
 Open