(VAR X Y X1 X2)
(RULES 
active(nats) -> mark(adx(zeros))
active(zeros) -> mark(cons(0,zeros))
active(incr(cons(X,Y))) -> mark(cons(s(X),incr(Y)))
active(adx(cons(X,Y))) -> mark(incr(cons(X,adx(Y))))
active(hd(cons(X,Y))) -> mark(X)
active(tl(cons(X,Y))) -> mark(Y)
mark(nats) -> active(nats)
mark(adx(X)) -> active(adx(mark(X)))
mark(zeros) -> active(zeros)
mark(cons(X1,X2)) -> active(cons(X1,X2))
mark(0) -> active(0)
mark(incr(X)) -> active(incr(mark(X)))
mark(s(X)) -> active(s(X))
mark(hd(X)) -> active(hd(mark(X)))
mark(tl(X)) -> active(tl(mark(X)))
adx(mark(X)) -> adx(X)
adx(active(X)) -> adx(X)
cons(mark(X1),X2) -> cons(X1,X2)
cons(X1,mark(X2)) -> cons(X1,X2)
cons(active(X1),X2) -> cons(X1,X2)
cons(X1,active(X2)) -> cons(X1,X2)
incr(mark(X)) -> incr(X)
incr(active(X)) -> incr(X)
s(mark(X)) -> s(X)
s(active(X)) -> s(X)
hd(mark(X)) -> hd(X)
hd(active(X)) -> hd(X)
tl(mark(X)) -> tl(X)
tl(active(X)) -> tl(X)
)