There exist a structure on double categories due to R.Brown called a connection. The connection embodies in squares an isomorphism between the category of its vertical arrows and the category of its horizontal arrows and it allows to change boundaries of a square from one type to the other. It generates a double subcategory that is very interesting to me. I was wondering if anyone had thought of and found a way to describe this subcategory without the mention of "commutative squares" or the construction of the connection itself. I have the feeling that it is given by an adjunction but it seems to elude me.
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