elisp mapcat

;; This is an elisp verision of the mapcat function found in the core Clojure library.

(defun mapcat (f xs &rest xss)
  "Returns the result of applying append to the result of applying mapcar
to f and sequences. Thus function f should return a sequence."
  ;; Make sure all the arguments are sequences.
  (assert
   (and (sequencep xs)
        (cl-every
         (lambda (x) (or (listp x) (vectorp x)))
         xss)))
  ;; Combine the top level sequences into one.
  (apply
   #'append
   ;; Run the supplied function on the sequence(s).
   (apply
    #'cl-mapcar
    f
    ;; Combine the sequences.
    (cons xs xss))))