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Rebase infrastructure for pushouts to span diagrams #885

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EgbertRijke
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In this PR I refactor the definitions of spans, morphisms of spans, and equivalences of spans (previously called homotopies of spans), and I will work on a better integration of this infrastructure in the theory of pushouts.

Since this touches upon earlier work of @VojtechStep and @tomdjong, perhaps you two can have a look at this PR when it's ready. (That's not to exclude @fredrik-bakke from also having a look)

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@EgbertRijke do you have a timeline in mind for finishing this PR? I'm not sure if I should be continuing my development with the old infrastructure, or if it's feasible to start using the shiny new spans

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Ok let me get it to a decent state so that you can start using it. I’ll try to get it done by the weekend.

@VojtechStep
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Great! Thanks

Comment on lines 419 to 423
cone-family-dependent-pullback-property P γ =
pair
( precompose-lifts P (horizontal-map-cocone-span s c) γ)
( pair
( precompose-lifts P (vertical-map-cocone-span s c) γ)
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copattern match

@EgbertRijke
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I'm converting span-fixed-domain-codomain to span and span to span-diagram. This should make our terminology more in use with the common mathematical use of a span, which is from one given object to another. Furthermore, cocones will be on span diagrams, which is analogous to cocones on sequential diagrams, so a recurring scheme is emerging.

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In order to complete this refactoring I'm gonna have to deal with pointed spans as well. Please hang on tight everyone:P

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EgbertRijke commented Jan 2, 2024

...and morphisms of pointed arrows too

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I am closing this pull request now, as I am not expecting to make progress on this any time soon.

@EgbertRijke EgbertRijke closed this Apr 6, 2024
VojtechStep added a commit that referenced this pull request Apr 6, 2024
In anticipation of #885. The proof of the universal property of
coequalizers being preserved by equivalences of coforks got a bit
unwieldy, but it should become more conceptual once morphisms and
equivalences of cocones under spans are integrated (for now they exist
only in the other PR).
VojtechStep pushed a commit that referenced this pull request Apr 19, 2024
Adds strict β-laws for the standard pushouts in a new module
`synthetic-homotopy-theory.rewriting-pushouts`.

### Todo
- [x] Wait for #885.
- [x] ~Refactor postulates of universal properties to be phrased in
terms of coherently invertible maps.~
- [x] Add separate file for rewrites
`synthetic-homotopy-theory.rewriting-pushouts`.
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3 participants