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This PR adds a little lemma: If a category has binary products and coreflexive equalizers, then it has equalizers. As usual, this implication is dualized automatically.
This lemma in particular shows that the category of manifolds does not have coreflexive equalizers and reflexive coequalizers.
I also documented the decision (which has been done long ago, but it was never written down) that the implications are preferred in their "limit" variant.