[Merged by Bors] - feat: add splitOnP_append_cons

[BoltonBailey]

This PR adds a lemma for splitOnP, allowing expressions where a splitOnP on a concatenation to be better simplified. Also golfs a relevant consequence with the new lemma.

Related to google-deepmind/formal-conjectures#1120

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[github-actions]

PR summary b9fc957e0c

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

+ splitOnP_append_cons

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).

[jcommelin]

Thanks 🎉

bors merge

[mathlib-bors]

Pull request successfully merged into master.

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