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