"Interesting, I hadn't paid attention to the licensing terms. I thought anybody could do anything with the code, period."
Whoops, you just reminded me it isn't that bad. :) Per (4)(b), a closed-source project (or a project under a different license) is just fine as long as it doesn't interfere with a separate Arc installation and it doesn't call itself "Arc."
Playing out some scenarios...
Theoretically, I could invoke (4)(b) to continue development of Rainbow.js under the MIT license plus a few restrictions. In particular, any forks of Rainbow.js could not be named "Arc" or interfere with Arc, but they could be named (and/or interfere with) "Rainbow" or "Rainbow.js."
Interestingly, the Artistic License 2.0 grants an explicit patent license for the Copyright Holder's patents, but not for the Contributors' patents. Not that this is necessarily a problem; I like the MIT license, but it doesn't specifically mention patents at all.
It seems like I'm now okay with AL2.0. Effectively the only thing it prohibits is using the name "Arc" without respecting the original creators. That condition would pretty much be ensured by trademark law anyway, if Arc were a product being sold.
P.S.: I'm not a lawyer. As if that weren't enough, I've demonstrated an ability to misunderstand this license over and over and over. :)
