Fix nix-shell

[CohenCyril]

Motivation for this change

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.

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[affeldt-aist]

fyi, the error with 9.0-master (pull_request)

File "./normedtype_theory/normed_module.v", line 104, characters 10-30:
Error: The reference Num.SubNormedZmodule was not found in the current
environment.

[affeldt-aist]

Can't merge yet because of conflicts