Here's the thing: I don't "get" setf-expanders and would like to learn how they work.
I need to learn how they work because I've got a problem which seems like a typical example for why you should learn setf-expanders, the problem is as follows:
(defparameter some-array (make-array 10))
(defun arr-index (index-string)
(aref some-array (parse-integer index-string))
(setf (arr-index "2") 7) ;; Error: undefined function (setf arr-index)
How do I write a proper setf-expander for ARR-INDEX?