Well, it's perhaps not only the language subset, but also part of the standard library you have to provide or re-implement. I don't know Lisp too well, but I'm

Well, it's perhaps not only the language subset, but also part of the standard library you have to provide or re-implement. I don't know Lisp too well, but I'm sure there are parts of it that are not necessarily mandatory for creating a prototype of a garbage collector. As for C#, the things we don't support are: - yield statements: codegen for that is not fun to do on your own and hand-writing an IEnumerator is annoying, but not too bad. And back when we started we couldn't get the generated code from Roslyn, maybe there's a way by now to do that. - async/await (could nowadays be converted into pretty much the same in JS at least, but Task/Promise/CompletableFuture work slightly differently anyway and there aren't that many places in the codebase that need this, so they can be patched) - dynamic (no reason to use it, thus no reason to support it). - Some generic constraints, e.g. new(): We currently emit JS, TypeScript, Java, and Python, along with a number of internally-relevant output formats like documentation – and .NET generics already don't map well to Java generics (or vice-versa), and are mostly useless for JS and Python, anyway. - goto, unless for fall-through in switch Most “normal” things like types/members/statements/expressions do work properly and the subset isn't too bad working in (it's also not a particularly small subset, of course). A lot of additional work comes in getting the necessary parts of the BCL to work on the other side of the conversion, of course, but that is a separate concern next to language features.