Have to watch out for corner cases - Arc's usage of 'nil as the list terminator is exposed in several places within the language itself. For example `(is (coerce "nil" 'sym) (cdr nil))` is true under the existing implementation, yet would be false if you just (defined nil '()). You'd need to patch `is`, `coerce`, and possibly a bunch of other primitives to preserve the existing behavior.
Which exposes the problem of "the code is the spec". If there was a prose spec, pg could have just said that such stuff was undefined and we wouldn't care that such things break.
If stuff doesn't work right, having the spec say it's supposed to not work right doesn't help at all.
Prose specs came into being to help multiple implementations interoperate (somewhat) when implemented in low level languages. I don't see the need when you have just one pretty concise and declarative implementation.
