[gentoo-user] Gobbledegook error message from emerge.

[Alan Mackenzie]

Hello, Gentoo.

I'm angry, because I've wasted several hours trying to decypher a wholly
unintuitive error "message" from emerge.  Because of this, it's been too
long since I've been able to update.  Help, please!

The command I typed in was

    # time emerge -auND @world.

I got back a horrendously long list of packages to merge (most of them
re-merges), followed by:

emerge: there are no ebuilds built with USE flags to satisfy "dev-python/pycparser[python_targets_python3_6(-)?,python_targets_python3_7(-)?,python_targets_python3_8(-)?,python_targets_python3_9(-)?,-python_single_target_python3_6(-),-python_single_target_python3_7(-),-python_single_target_python3_8(-),-python_single_target_python3_9(-)]".
!!! One of the following packages is required to complete your request:
- dev-python/pycparser-2.20-r1::gentoo (Change USE: +python_targets_python3_7)
- dev-python/cffi-1.14.0-r3::gentoo (Change USE: -python_targets_python3_7)
(dependency required by "dev-python/cffi-1.14.0-r3::gentoo" [ebuild])
(dependency required by "dev-python/bcrypt-3.2.0::gentoo" [installed])
(dependency required by "dev-python/paramiko-2.7.1::gentoo" [installed])
(dependency required by "dev-vcs/bzr-2.7.0::gentoo" [installed])
(dependency required by "@selected" [set])
(dependency required by "@world" [argument])

.  What is the syntax and semantics of that first line?  Is it explained
somewhere in the emerge manual?  What do the square brackets mean?  What
do the commas within these square brackes mean?  AND?  OR?  What about
the question marks following minus signs inside parentheses?

What does "(Change USE: +python_targets_python3_7)" mean?  Is it a
suggestion to me that I should change some USE flag somewhere?  Or is it
telling me that a changed USE flag is the reason for all the trouble?  Or
does it mean something else?

What is "dependency required by" supposed to mean?  I can understand a
dependency of A on B.  I can understand A being required by B.  But a
dependency being required?  what does this mean?

Presumably to get out of the mess I've got to do something with one of
the packages which "is required to complete your request".

Surely somebody in the Gentoo portage team must see that printing human
readable error messages is a worthwhile thing to do.  Surely?

-- 
Alan Mackenzie (Nuremberg, Germany).