This works, but I was vaguely hoping to do it via the macro system (so that the produced content is identical to the effect of typing \cdots or whatever). Of course, I haven't had time to do that yet, so maybe this is worth going with...

I don't think our support for macros is powerful enough b/c we need to know what the next token is and then somehow use that to decide which version of dots to render.

Yes, but I think it'd be worth adding support for that to the macro parser. I wasn't planning on implementing \futurelet, but rather allow defineMacro to give a function which gets a context that has a nextToken equivalent of \futurelet... Of course, at that point, maybe I'm just implementing defineFunction?

Maybe the dots function should recursively call the parser to get the output of \cdots and \ldots? Is that crazy?