(VAR X Y )
(RULES 
        tail(cons(X)) -> Y
        if(true) -> X
        if(false) -> Y
        primes -> sieve(from(s(s(0))))
        from(X) -> cons(X)
        head(cons(X)) -> X
        filter(s(s(X)), cons(Y)) -> if(divides(s(s(X)), Y))
        sieve(cons(X)) -> cons(X)
        
)