(VAR X Y X1 X2)
(RULES 
active(from(X)) -> mark(cons(X,from(s(X))))
active(length(nil)) -> mark(0)
active(length(cons(X,Y))) -> mark(s(length1(Y)))
active(length1(X)) -> mark(length(X))
mark(from(X)) -> active(from(mark(X)))
mark(cons(X1,X2)) -> active(cons(mark(X1),X2))
mark(s(X)) -> active(s(mark(X)))
mark(length(X)) -> active(length(X))
mark(nil) -> active(nil)
mark(0) -> active(0)
mark(length1(X)) -> active(length1(X))
from(mark(X)) -> from(X)
from(active(X)) -> from(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)
s(mark(X)) -> s(X)
s(active(X)) -> s(X)
length(mark(X)) -> length(X)
length(active(X)) -> length(X)
length1(mark(X)) -> length1(X)
length1(active(X)) -> length1(X)
)