The base idea is pretty good, although the 'cons cell (which is composed of a type id, a car pointer, and a cdr pointer) can be replaced with a smaller "closure-settable" cell, which would be an untyped object composed of a single pointer:
(fn (x)
; % means it's a primitive
(let x (%closure-settable x)
(list
(fn () (%closure-settable-read x))
(fn (v) (%closure-settable-write x v)))))
Because the closure variable abstraction is not accessible from the high-level Arc language, the closure-settable doesn't need to be typed tagged
