Yeah it lets you decide what to do when getting or setting a variable. The original link has an example at the bottom, but here's another one kinda related to what you seem to be trying to do:
arc> (= h (obj a 1 b 2))
#hash((a . 1) (b . 2))
arc> (defvar a
(fn args
(if args
; write
(= h!a car.args)
; read
h!a)))
arc> a
1
arc> (= a 3)
3
arc> a
3
arc> h
#hash((a . 3) (b . 2))
