Guix,

bricewge on #guix informed me that there's no way to pass a git 
tag as source transformation option.  Indeed:

  ‘--with-commit=PACKAGE=COMMIT’
     This is similar to ‘--with-branch’, except that it builds 
     from
     COMMIT rather than the tip of a branch.  COMMIT must be a 
     valid Git
     commit SHA1 identifier.

is quite different from (guix git-download)'s pleasantly liberal 
notion of commit:

  (git reference
    (url "git://foo.org/fizbo")
    (commit "fizbo-4.5"))      ; tag yay

bricewge suggested that a single ‘--with-git-ref[erence]=’ could 
replace both ‘--with-branch’ and a new ‘--with-tag’, and I agree. 
(Although I prefer the full spelling, of course :-)

Two questions:
- Is this really not supported yet, or am I missing the obvious?
- Why is the (extremely) git-specific ‘--with-commit=’ option not 
  called ‘--with-git-commit=’?  Was it intended to be more generic 
  than it is now?  Should the new option be ‘--with-reference=’ as 
  well?  That's pushing it a little far.  And three questions in 
  one; I'm cheating.

Kind regards,

T G-R