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quotient groups
Wouldn't it make sense for quotient groups to be defined in the way strictly dual to the way subgroups are defined? Here are the two definitions:
Then the connection with normal subgroups would be drawn.
And quotient groups could have their own chapter, too, since subgroups do.
In other words, I propose changing "set of surjections" to "type of quotient groups".
And also starting more gently, by saying that a quotient group of G is an epimorphism from G to a group.
By the way, it seems to be left unproved that the set of surjections from G is a set.