[Merged by Bors] - feat(Data/List): add theorem length_eq_two'

[IvanRenison]

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[github-actions]

PR summary 9b809f3970

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

+ length_eq_two'

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 scripts/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).

[vihdzp]

Can grind solve this? Perhaps rw [length_eq_two]; grind?

[vihdzp]

I wonder if the statement

theorem length_eq_two' {l : List α} : l.length = 2 ↔ ∃ h, l = [l.head h, l.getLast h] := ...

might be more useful? That's up to you, otherwise LGTM.

[IvanRenison]

In my use case in #35777 it does not work

[Vierkantor]

I would even mildly prefer this phrasing over the original length_eq_two (since replacing ∃ with explicit witnesses is nicer!). So looks good to me!

bors r+

[mathlib-bors]

This PR was included in a batch that was canceled, it will be automatically retried

[mathlib-bors]

Pull request successfully merged into master.

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