Taking as a starting point the author's previous work on developing aspects of category theory in Isabelle/HOL, this article gives a compatible formalization of the notion of "bicategory" and develops a framework within which formal proofs of facts about bicategories can be given. The framework includes a number of basic results, including the Coherence Theorem, the Strictness Theorem, pseudofunctors and biequivalence, and facts about internal equivalences and adjunctions in a bicategory. As a driving application and demonstration of the utility of the framework, it is used to give a formal proof of a theorem, due to Carboni, Kasangian, and Street, that characterizes up to biequivalence the bicategories of spans in a category with pullbacks. The formalization effort necessitated the filling-in of many details that were not evident from the brief presentation in the original paper, as well as identifying a few minor corrections along the way.
Revisions made subsequent to the first version of this article added additional material on pseudofunctors, pseudonatural transformations, modifications, and equivalence of bicategories; the main thrust being to give a proof that a pseudofunctor is a biequivalence if and only if it can be extended to an equivalence of bicategories.
Move ConcreteCategory.thy from Bicategory to Category3 and use it systematically.
Make other minor improvements throughout.
[2020-11-04] Added new material on equivalence of bicategories, with associated changes. (revision 472cb2268826)
[2021-07-22] Added new material "concrete bicategories" and "bicategory of categories". (revision 49d3aa43c180)