(VAR x xs y )
(RULES 
        add(true, x, xs) -> add(and(isNat(x), isList(xs)), x, Cons(x, xs))
        isList(Cons(x, xs)) -> isList(xs)
        isList(nil) -> true
        isNat(s(x)) -> isNat(x)
        isNat(0) -> true
        if(true, x, y) -> x
        if(false, x, y) -> y
        and(true, true) -> true
        and(false, x) -> false
        and(x, false) -> false
        
)