MiniKanren twinso

(define twinso
  (lambda (s)
    (fresh (x y)
      (conso x y s) ;; s is a list of 2 elements, x `cons` (y)
      (conso x () y)))) ;; these elements should be the same

Without conso

(define twinso
  (lambda (s)
    (fresh (x)
      (== '(x x) s))))