(COMMENT digits = [0,..,9])
(VAR x y)
(RULES
 digits -> d(0)
 d(x) -> if(le(x,s(s(s(s(s(s(s(s(s(0)))))))))),x)
 if(true,x) -> cons(x,d(s(x)))
 if(false,x) -> nil

 le(0,y) -> true
 le(s(x),0) -> false
 le(s(x),s(y)) -> le(x,y)
)