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This is just to suggest another “test problem”, concerning a question implicity asked in the thread
namely how to professionally/routinely/usually handle the fact that according to the usual conventions, one may arbitrarily modify diagrammatically-given axioms by introducing identity arrows into them, and then the resulting diagram, while of course dismissable by “nothing happened, you did the category-theoretic equivalent of a Reidemeister move”, can “trigger” different category-theoretic concepts/ideas: I could point to at least one literature reference which treats the axioms for
by displaying the parallel pair of functors
cod⟶D1D0dom⟶as the span of functors
D0----------------------------------------D0cod↖dom↗D1which then may “trigger” the concept of pushouts-in-CAT, and another literature reference which arbitrarily”blows-up” the parallel pair of functors into a cospan of functors
D1----------------------------------------D1cod↘dom↙D0which may “trigger” the concept of pullbacks-in-CAT.
One of the references actually uses pullbacks-in-CAT in the sequel .
I do not expect there to be much to be said, in particular since pushouts and pullbacks are dual concepts (which in turn is reflected in how small the “edit-distance” from the above code for the above span into the above code of the above cospan is) so that here in a sense even less happens than in the case of the example of monoidal bicategories axiomatized with, alternatively, a triangular or a quadrangular μiddle unitor. Except for some evident things, such as that a reasonable definition of Mod() should judge
Mod(axiomatization of double-categories with a parallel pair)
“equal” to
Mod(axiomatization of double-categories with a span)
“equal” to
Mod(axiomatization of double-categories with a cospan)
and therefore must somehow always simultaneously be “aware” of both sides of any two dual cat-concepts.
Again, there probably is not much to say here, I am just floating this particularly “trivial” and symmetric and self-dual example. But who knows?
2-categorical algebra is very well-studied. 2-monads are one aspect of it, also there are 2-algebraic theories and so on. I’m not sure what you mean by “trigger”, but if you just mean that drawing things in a different way may make the reader think of different ideas, then that has nothing to do with categorification.
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