[Merged by Bors] - chore: don't unfold Pi.single in a simp lemma

[eric-wieser]

Doing this prevents the many lemmas we have about Pi.single applying.

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[github-actions]

PR summary 725d7a32b7

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff

No declarations were harmed in the making of this PR! 🐙

You can run this locally as follows

## summary with just the declaration names:
./scripts/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/declarations_diff.sh long <optional_commit>

The doc-module for script/declarations_diff.sh contains some details about this script.

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No changes to technical debt.

You can run this locally as

./scripts/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).

[themathqueen]

I think we should always use Pi.single instead of the bare functions one (regardless if it's a simp imo). Are there any more of these? I remember a PR a few months back that changed some to use Pi.single. I don't think it ever got merged yet, let me try finding it (edit: #31949).

[eric-wieser]

I generally agree, but I don't think we need to make that change all at once.

[b-mehta]

Thanks!

bors merge

[mathlib-bors]

Pull request successfully merged into master.

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