How to select git:send-email output in config.scm?

[EuAndreh <eu@euandre.org>]

Hi guix!

How can I selecet the git:send-email output in my "operating-system"
declaration inside config.scm?

Right now I only have the git package, but I don't know which output it
is selecting. I guess the git:out one?

I tried adding it just like that to my config.scm declaration:

--8<---------------cut here---------------start------------->8---
  (packages
    (append
      (map specification->package
           '("git"
             "git:send-email"
             ...)
--8<---------------cut here---------------end--------------->8---

But when running guix system reconfigure, it complains about it, saying:

--8<---------------cut here---------------start------------->8---
guix system: error: git:send-email: unknown package
guix system: error: failed to load 'guix.scm':
ice-9/boot-9.scm:222:17: In procedure map1:
--8<---------------cut here---------------end--------------->8---

However, running "guix environment --ad-hoc git:send-email" does work.

I got a bit confused by that. On IRC pkill9 mentioned that "git" is a
package, but "git:send-email" is an output. But isn't "git" the same as
"git:out"?

How can I declare that I want the "git:send-email" output also installed
in my config.scm?