(VAR F H T X )
(RULES 
        rand(X) -> X
        rand(s(X)) -> rand(X)
        bool(0) -> false
        bool(s(0)) -> true
        filter(F, nil) -> nil
        filter(F, cons(H, T)) -> consif(, H, filter(F, T))
        consif(true, H, T) -> cons(H, T)
        consif(false, H, T) -> T
        
)