(VAR L N X X1 X2 )
(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)
        
)