proof of concept: simp lemmas spawing subgoals

[datokrat]

TBA

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 96d502bd11cc96bd92dcb6b73e1c4002526647ca --onto 4f6bcc5adac8d6234a9e3def3675402cb89823c6. You can force Mathlib CI using the force-mathlib-ci label. (2026-04-10 12:59:58)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 96d502bd11cc96bd92dcb6b73e1c4002526647ca --onto 861bc19e0c1c45b5766cc5c9ef0488568368b236. You can force reference manual CI using the force-manual-ci label. (2026-04-10 13:00:00)