(VAR x y )
(RULES 
        p(0) -> s(s(0))
        p(s(x)) -> x
        p(p(s(x))) -> p(x)
        le(p(s(x)), x) -> le(x, x)
        le(0, y) -> true
        le(s(x), 0) -> false
        le(s(x), s(y)) -> le(x, y)
        minus(x, y) -> if(le(x, y), x, y)
        if(true, x, y) -> 0
        if(false, x, y) -> s(minus(p(x), y))
        
)