(VAR L N X1 X2 X)
(RULES 
active(zeros) -> mark(cons(0,zeros))
active(U11(tt,L)) -> mark(U12(tt,L))
active(U12(tt,L)) -> mark(s(length(L)))
active(length(nil)) -> mark(0)
active(length(cons(N,L))) -> mark(U11(tt,L))
mark(zeros) -> active(zeros)
mark(cons(X1,X2)) -> active(cons(mark(X1),X2))
mark(0) -> active(0)
mark(U11(X1,X2)) -> active(U11(mark(X1),X2))
mark(tt) -> active(tt)
mark(U12(X1,X2)) -> active(U12(mark(X1),X2))
mark(s(X)) -> active(s(mark(X)))
mark(length(X)) -> active(length(mark(X)))
mark(nil) -> active(nil)
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)
U11(mark(X1),X2) -> U11(X1,X2)
U11(X1,mark(X2)) -> U11(X1,X2)
U11(active(X1),X2) -> U11(X1,X2)
U11(X1,active(X2)) -> U11(X1,X2)
U12(mark(X1),X2) -> U12(X1,X2)
U12(X1,mark(X2)) -> U12(X1,X2)
U12(active(X1),X2) -> U12(X1,X2)
U12(X1,active(X2)) -> U12(X1,X2)
s(mark(X)) -> s(X)
s(active(X)) -> s(X)
length(mark(X)) -> length(X)
length(active(X)) -> length(X)
)