(VAR b x y )
(RULES 
        minus(x, x) -> 0
        minus(s(x), s(y)) -> minus(x, y)
        minus(0, x) -> 0
        minus(x, 0) -> x
        div(s(x), s(y)) -> s(div(minus(x, y), s(y)))
        div(0, s(y)) -> 0
        f(x, 0, b) -> x
        f(x, s(y), b) -> div(f(x, minus(s(y), s(0)), b), b)
        
)