gistfile1.scm

;; Take a list of (possibly repeated) elements, and return all those
;; that appear more than once.

(include "~/repos/simple-miniKanren/mkdefs.scm")

;; Regular Scheme version for comparison
(define morethanonce
  (lambda (ls)
    (cond
      [(null? ls) '()]
      [(member (car ls) (cdr ls))
       (cons (car ls) (morethanonce (cdr ls)))]
      [else (morethanonce (cdr ls))])))

(define morethanonceo
  (lambda (ls out)
    (fresh (a d res)
      (conde
        [(== '() ls) (== '() out)]
        [(caro ls a)
         (cdro ls d)
         (membero a d)
         (morethanonceo d res)
         (conso a res out)]
        [(cdro ls d)
         (morethanonceo d out)]))))