fix: forgotten lemma about derivability of sqrt

[affeldt-aist]

Motivation for this change

Should the naming be rather:

derivable_sqrt -> derivable1_sqrt

and

derive_sqrt -> derive1_sqrt

? @CohenCyril @holgerthies

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

• added corresponding documentation in the headers

Reference: How to document

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