Get lsb d get elem simp

[tobiasgrosser]

Read this section before submitting

• Ensure your PR follows the External Contribution Guidelines.
• Please make sure the PR has excellent documentation and tests. If we label it missing documentation or missing tests then it needs fixing!
• Include the link to your RFC or bug issue in the description.
• If the issue does not already have approval from a developer, submit the PR as draft.
• The PR title/description will become the commit message. Keep it up-to-date as the PR evolves.
• A toolchain of the form leanprover/lean4-pr-releases:pr-release-NNNN for Linux and M-series Macs will be generated upon build. To generate binaries for Windows and Intel-based Macs as well, write a comment containing release-ci on its own line.
• If you rebase your PR onto nightly-with-mathlib then CI will test Mathlib against your PR.
• You can manage the awaiting-review, awaiting-author, and WIP labels yourself, by writing a comment containing one of these labels on its own line.
• Remove this section, up to and including the --- before submitting.

---

Closes #0000 (RFC or bug issue number fixed by this PR, if any)

[tobiasgrosser]

This has been merged months ago.