(defn greet-me  (print "hey programmer!"))
(defn greet-stranger  (print "hey stranger!"))
(varfn greet  (greet-me))
(greet) # prints "hey programmer!"
(varfn greet  (greet-stranger))
(greet) # prints "hey stranger!"
# kind of analogous to (def greet (fn  (some-thunk))), but with built-in
# sugar to automatically update 'greet' if varfn is called with a name
# already bound