In this implementation an ordered user-defined set is a list with 'ordered as its first element and a vector as its cadr. A Sol ordered set evaluates (in the sense of eval) to itself.
(define (ordered . a)
(if (null? a)
(list 'ordered (vector))
(let ((vect (car a)))
(if (vector? vect)
(list 'ordered vect )
(list 'ordered (list->vector a))))))