{- 1st part checks all of [a] is in first part of [(a,b)] and that there is no a in [(a,b)] which is not in [a] i.e. subsets of each other and equal to one another 2nd part first removes any exact ...

We’re brought up to say that the dual concept of injection is surjection, and of course there’s a perfectly good reason for this. The monics in the category of sets are the injections, the epics are ...