defining setf-expanders in Common Lisp

Question

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

Answer 1

Rainer's answer is spot on. Before ANSI Common Lisp, it was necessary to use defsetf to define an expander for simple places that could be set with a simple function call. setf functions like (setf arr-index) came into the language with CLOS and simplify a lot of things. In particular, setf functions can be generic.