gistfile1.clj

(defn nullo
  [x]
  (== '() x))

(defn pairo
  [x]
  (fresh [a b]
    (conso a b x)))

(defn flatteno
  "Relation where out is a flattened s"
  [s out]
  (conde
    ((nullo s) (== () out))
    ((pairo s)
      (fresh [a d res-a res-d]
        (conso a d s)
        (flatteno a res-a)
        (flatteno d res-d)
        (appendo res-a res-d out)))
    ((conso s () out))))

(run 1 [q]
   (flatteno [1 [2 3] [4]] q)) => (([1 [2 3] [4]]))