derive_mx

[affeldt-aist]

Motivation for this change

two lemmas from robot-rocq that are also useful for ode @CohenCyril

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

• added corresponding documentation in the headers

Reference: How to document

Merge policy

As a rule of thumb:

• PRs with several commits that make sense individually and that
  all compile are preferentially merged into master.
• PRs with disorganized commits are very likely to be squash-rebased.

Reminder to reviewers

• Read this Checklist
• Put a milestone if possible
• Check labels

[affeldt-aist]

@holgerthies @yosakaon @IshiguroYoshihiro if one of you as a minute, could you take a quick look?

Since theses lemmas have already by tested by others and proved useful elsewhere, I think that it is reasonable to merge them if CI is green and if there are no objections.