Subfunctor.lean`

Explanation legend Lean4 Domain Explanation Language (L4DEL)

L4DEL explanations are a separate layer. They do not change the Lean source. Intuition is explanatory prose; numbered items are a reading path; sources link the wording back to evidence.

L4DEL: Lean4 Domain Explanation Language

Annotation provenance. This overlay contains 8 curated AI-written annotations and 1204 mechanical source-derived annotations. They have not been human-reviewed as explanations; treat them as unverified and not mathematical authority. Retained generation history: The 71 original curated annotations were produced in a Claude Sonnet 5.5 session at medium effort; Claude Opus 5.5 is recorded in that session as the advisor model, not the direct author. The bulk source-derived expansion was generated mechanically from pinned source comments by tooling developed in a GPT-5.6 Sol session at ultra reasoning; GPT-5.6 Sol did not originate the quoted source prose. On 2026-10-17 the mechanically generated source-comment annotations of the Formal Conjectures, Carleson, FLT, PFR, CompFiles, Mathematics in Lean and Theorem Proving in Lean overlays were regenerated by the same generator after three corrections made in a Claude Sonnet 5.5 session: titles no longer cut a sentence inside mathematics or end on a dangling word, workflow markers are removed from the quoted prose, and declaration names follow the scanner's namespace resolution. The boilerplate step line was dropped, and comments that were only a row of dashes no longer yield annotations. No new prose was written and no human review is claimed.

Credit: The upstream authors and contributors retain credit for the project and mathematics. lean4-explainer only renders their pinned source and adds the separately disclosed explanation layer.

Project sources: Pinned project documentation, Pinned FLT blueprint introduction

1 declarations, 0 contain sorry, 0 declared as axioms

Subfunctors

Basic constructions for subfunctors of functors valued in Type, including the subfunctor cut out by a subset of the value at a terminal object.

Imports (2)

Imported by 2 files

def CategoryTheory.Subfunctor.ofIsTerminal The subfunctor defined by pulling back a subset of the terminal component line 30 source
Source documentation
The subfunctor defined by pulling back a subset of the terminal component.

standard axioms: Classical.choice, Quot.sound, propext what this means
Statement uses: CategoryTheory.Category, CategoryTheory.Functor, CategoryTheory.Limits.IsTerminal, Set, CategoryTheory.Functor.obj, CategoryTheory.Subfunctor
Proof uses: CategoryTheory.Subfunctor.mk, Set.preimage, TypeCat.Fun, CategoryTheory.ConcreteCategory.hom, CategoryTheory.Functor.map, CategoryTheory.Limits.IsTerminal.from, Quiver.Hom
8 instances, coercions and auxiliary constants left out.

Tactic trace: 2 steps
Open to load the checked before-and-after goal states.
```
def ofIsTerminal {X : C} (hX : Limits.IsTerminal X) (s : Set (F.obj X)) :
    Subfunctor F where
…
```