Preunivalence implies strong preunivalence #1364
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This PR currently contains a postulate for the 3-for-2 property for isomorphisms. @EgbertRijke has formalizations for split epi-/monomorphisms in precategories in an unmerged branch (#1071), so I'll leave that for whenever those are merged. EDIT: Never mind, I found a direct proof |
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I found a simple route to prove the last lemma, so this PR is ready for review. |
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As demonstrated by Evan Cavallo, strong preunivalence follows from preunivalence + function extensionality.
By an analogous proof, preunivalent categories are strongly preunivalent.