> In addition to requiring 'geiser-install you have to set the
> geiser-active-implementations variable, e.g.:

Customizing the geiser-active-implementations variable works for
me. Thank you.

> alternatively there is a :scheme header argument for src blocks:
>
> #begin_src scheme :scheme guile

Unfortunately, this doesn't work for me. Is this header argument
documented somewhere in the manual?

This approach seems more self-contained to the org file, and it'll be
good to have this, in case I want to send my org file to someone else,
or if I decide to try out more scheme implementations.