WS Error! on-open (void-function seq-map-indexed) about indium HOT 2 CLOSED

;; HACK: In Emacs 25.1, an older version of seq.el is provided, which can be
;; loaded before jade or even package.el.  If this happens, the feature `seq'
;; being already provided, the correct version of seq.el won't get loaded.

(defun seq-map-indexed (function sequence)
    "Return the result of applying FUNCTION to each element of SEQUENCE.
Unlike `seq-map', FUNCTION takes two arguments: the element of
the sequence, and its index within the sequence."
    (let ((index 0))
      (seq-map (lambda (elt)
                 (prog1
                     (funcall function elt index)
                   (setq index (1+ index))))
               sequence)))