(VAR X Y )
(RULES 
        fact(X) -> if(zero(X), s(0), prod(X, fact(p(X))))
        add(0, X) -> X
        add(s(X), Y) -> s(add(X, Y))
        prod(0, X) -> 0
        prod(s(X), Y) -> add(Y, prod(X, Y))
        if(true, X, Y) -> X
        if(false, X, Y) -> Y
        zero(0) -> true
        zero(s(X)) -> false
        p(s(X)) -> X
        
)