[Merged by Bors] - feat(CategoryTheory/ComposableArrows): API for the composition of four arrows

[joelriou]

This shall be used in the formalization of spectral sequences #33842.

---

• depends on: [Merged by Bors] - feat(CategoryTheory/ComposableArrows): isIso_iff #33879
• depends on: [Merged by Bors] - feat(CategoryTheory/ComposableArrows): API for the composition of three arrows #33880

[github-actions]

PR summary a720914be0

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference
Mathlib.CategoryTheory.ComposableArrows.Four (new file)	448

---

Declarations diff

+ fourδ₁Toδ₀
+ fourδ₁Toδ₀'
+ fourδ₁Toδ₀_app_one
+ fourδ₁Toδ₀_app_three
+ fourδ₁Toδ₀_app_two
+ fourδ₁Toδ₀_app_zero
+ fourδ₂Toδ₁
+ fourδ₂Toδ₁'
+ fourδ₂Toδ₁_app_one
+ fourδ₂Toδ₁_app_three
+ fourδ₂Toδ₁_app_two
+ fourδ₂Toδ₁_app_zero
+ fourδ₃Toδ₂
+ fourδ₃Toδ₂'
+ fourδ₃Toδ₂_app_one
+ fourδ₃Toδ₂_app_three
+ fourδ₃Toδ₂_app_two
+ fourδ₃Toδ₂_app_zero
+ fourδ₄Toδ₃
+ fourδ₄Toδ₃'
+ fourδ₄Toδ₃_app_one
+ fourδ₄Toδ₃_app_three
+ fourδ₄Toδ₃_app_two
+ fourδ₄Toδ₃_app_zero

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.

---

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).

[mathlib4-merge-conflict-bot]

This pull request has conflicts, please merge master and resolve them.

[mathlib4-dependent-issues-bot]

This PR/issue depends on:

• [Merged by Bors] - feat(CategoryTheory/ComposableArrows): isIso_iff #33879
• [Merged by Bors] - feat(CategoryTheory/ComposableArrows): API for the composition of three arrows #33880
  By Dependent Issues (🤖). Happy coding!

[dagurtomas]

Thanks!

maintainer merge

[github-actions]

🚀 Pull request has been placed on the maintainer queue by dagurtomas.

[jcommelin]

Thanks 🎉

bors merge

[mathlib-bors]

Pull request successfully merged into master.

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