I have basic macros working fine.
I would like to implement block macros / syntactic transforms,
but not just yet!

I will continue simplifying the coroutine syntax as much as
possible using this macro system.

TODO automatically wrap expression macros in parentheses?

NOTE my macros are unhygenic w.r.t. argument names, e.g:

define r(foo) (f->foo-r)

main()
	r(foo)

recurses infinitely because we are substituting foo->foo

perhaps we shouldn't expand macros after substituting the
arguments, but only before.  yes.