;;; Each structure that may be placed in a SSet must include the
;;; SSet-Element structure. We allow an initial value of NIL to mean
;;; that no ordering has been assigned yet (although an ordering must
;;; be assigned before doing set operations.)
;;; Each structure that may be placed in a SSet must include the
;;; SSet-Element structure. We allow an initial value of NIL to mean
;;; that no ordering has been assigned yet (although an ordering must
;;; be assigned before doing set operations.)