> With the recently-added ‘package-output’ (singular):

> scheme@(guile-user)> (package-output s binutils "out")
> $2 = "/nix/store/bfh5c2a4is27jdmc811fp6g0jfac7fiw-binutils-2.22"
> scheme@(guile-user)> (package-output s binutils "lib")
> $3 = "/nix/store/n29brzqlhjkzww51labk9anx493gl4d3-binutils-2.22-lib"

What's the value of 's' here?  There is no explanation in the docstrings
of 'package-derivation', 'package-cross-derivation', and
'package-output'.

Are you sure that it's OK to have 'package-output' and 'package-outputs'
in the same module?  Can we rename one?