>> +@defvr {Scheme Variable} pyproject-build-system
>> +This is a variable exported by @code{guix build-system pyproject}.  It
>> +is a reimplementation of @code{python-build-system} designed around
>                              ^
> s/code/var

Fixed both instances, thanks.  :-)

I also made some substantial changes to the build system:

* Moved 'python-toolchain' into python.scm which seems to work fine.
* Inherit from python-build-system where possible.
* Hint about deprecating 'add-installed-pythonpath', which has always
  annoyed me :-) I think a 'with-installed-pythonpath' would be much
  more ergonomic and plan to submit that later.
* Most importantly, use three ;;; for module commentary.

Patch below: