Halfway to anywhere, but the window to anywhere is quite small. For Mars, I think it's only a few weeks every two years or so. So having a lot of mass already halfway there when the window opens would be quite advantageous.
The "syntax extension" thing is precisely the reason I have an interest in (and advocacy for) Common Lisp.
Be that as it may, for C++, Eric Neibler's [Boost.Proto](https://www.boost.org/doc/libs/1_84_0/doc/html/proto.html) could likely help conveniently connect syntax to implementation to achieve something similar to what the author is taking about.