(VAR x y z u)

(RULES 
-(x,0)       -> x
-(s(x),s(y)) -> -(x,y)

<=(0,y)    -> true
<=(s(x),0)    -> false
<=(s(x),s(y)) -> <=(x,y)

if(true,  x, y) -> x
if(false, x, y) -> y

perfectp(0)    -> false
perfectp(s(x)) -> f(x, s(0), s(x), s(x))

f(0, y, 0, u)       -> true
f(0, y, s(z), u)    -> false
f(s(x), 0, z, u)    -> f(x, u, -(z,s(x)), u)
f(s(x), s(y), z, u) -> if(<=(x, y), f(s(x), -(y,x), z, u), f(x, u, z, u))
)
(COMMENT Perfect Numbers)