mo-compare-killring.el

(defun mo-compare-killring ()
  "copy some text to kill-ring, set point to the beginning of similar looking text, run this function.
The cursor will be set to the first difference"
  (interactive)
  (let ((s (current-kill 0 t))
	(pos 0))
    (while (and (< pos (length s))
		(equal (aref s pos) (char-after (point))))
      (forward-char)
      (incf pos))
    (if (< pos (length s))
        (message "clipboard NOT completely found!")
      (message "clipboard equal to text"))))

cl-incf statt incf