(VAR N X Y Z )
(RULES 
        terms(N) -> cons(recip(sqr(N)), terms(s(N)))
        sqr(0) -> 0
        sqr(s(X)) -> s(add(sqr(X), dbl(X)))
        dbl(0) -> 0
        dbl(s(X)) -> s(s(dbl(X)))
        add(0, X) -> X
        add(s(X), Y) -> s(add(X, Y))
        first(0, X) -> nil
        first(s(X), cons(Y, Z)) -> cons(Y, first(X, Z))
        half(0) -> 0
        half(s(0)) -> 0
        half(s(s(X))) -> s(half(X))
        half(dbl(X)) -> X
        
)