Append

append concatenates two lists to form a single list. The first argument is copied; the second argument is reused (shared).

(append '(a b c) '(d e))    ->  (a b c d e)

(defn append [x y]
  (if (empty? x)
      y
      (cons (first x)
            (append (rest x) y)) ) )

This version of append append is simple and elegant; it takes O(n_x) time and O(n_x) stack space. Time and stack space are independent of y, since y is reused but not processed.