##### Motivation for this change

<!-- if this PR fixes an issue, use "fixes #XYZ" -->

<!-- you may also explain what remains to do if the fix is incomplete -->

##### Things done/to do

<!-- please fill in the following checklist -->
- [ ] added corresponding entries in `CHANGELOG_UNRELEASED.md`
  (do not edit former entries, only append new ones, be careful:
   merge and rebase have a tendency to mess up `CHANGELOG_UNRELEASED.md`)
- [ ] added corresponding documentation in the headers
<!-- Cross-out the above items using ~crossed out item~ if they happen not to be relevant -->

<!-- leave this note as a reminder to reviewers -->
##### Automatic note to reviewers

Read [this Checklist](https://github.com/math-comp/math-comp/wiki/Checklist-for-following,-reviewing-and-playing-with-a-PR#checklist-for-reviewing-a-pr) and put a milestone if possible.
