(from AG01 4.26)
(VAR x y)
(RULES
p(0) -> 0
p(s(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))
)