(VAR X XS N X1 X2)
(RULES 
active(from(X)) -> mark(cons(X,from(s(X))))
active(after(0,XS)) -> mark(XS)
active(after(s(N),cons(X,XS))) -> mark(after(N,XS))
mark(from(X)) -> active(from(mark(X)))
mark(cons(X1,X2)) -> active(cons(mark(X1),X2))
mark(s(X)) -> active(s(mark(X)))
mark(after(X1,X2)) -> active(after(mark(X1),mark(X2)))
mark(0) -> active(0)
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)
after(mark(X1),X2) -> after(X1,X2)
after(X1,mark(X2)) -> after(X1,X2)
after(active(X1),X2) -> after(X1,X2)
after(X1,active(X2)) -> after(X1,X2)
)