This is the result of a light-bulb going on when reading an info file that introduces emacs lisp. I’m not sure this is legal in other lisps, and it goes against el camino of lisp. But it demonstrates what became clear to me…
(defun inc (x)
(set x (+ 1 (eval x)))) ;note the intentional use of set, not setq
(setq myvar 5)
(inc 'myvar)
(inc 'myvar)
(inc 'myvar)
(message "myvar's value: %s" myvar)