Clojure permutations implementation

(defn drop-nth
  "Removes the nth element from a collection."
  [sqn n]
  (concat (take n sqn) (drop (inc n) sqn)))

(defn drop-each
  "Produces a collection of copies of the input, each with a different element removed."
  [sqn]
  (map #(drop-nth sqn %)
       (range (count sqn))))

(defn cons-to-all
  "Exactly what it says on the tin."
  [x, sqns]
  (map #(cons x %) sqns))

(defn permutations
  "Produces a collection of all possible unique rearrangements of the input sequence."
  ;; FINALLY.
  [sqn]
  (if (= 1 (count sqn))
      #{sqn}
      (reduce into #{}
              (map #(cons-to-all %1 (permutations %2))
                   sqn
                   (drop-each sqn)))))

==================== interactive output ====================

mathdice-solver.core=> (permutations [1 2 3])
#{(2 3 1) (1 3 2) (2 1 3) (3 1 2) (1 2 3) (3 2 1)}
mathdice-solver.core=> (permutations [1 2 2])
#{(2 2 1) (2 1 2) (1 2 2)}
mathdice-solver.core=> (permutations [1 1 1])
#{(1 1 1)}