Flattening a nested list in clojure

(defn flat?
  "Returns true if seq contains no sequences"
  [seq]
  (not-any? (fn [x] (isa? (type x) java.util.List)) seq))

(defn flatten 
  "Returns an unnested sequence from the non-sequence elements of seq
for example, it turns (1 (2) 3) into (1 2 3)"
  [seq]
  (if (isa? (type seq) java.util.List)
    (if (flat? seq)
      seq
      (apply concat (map flatten seq)))
    (list seq)))