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Oh boy. Reading the comment in Issue9212 more carefully now that you said that I see that Mr. Hettinger said exactly that. I apologize for missing it the first time.
"Concrete classes are allowed to have more features than the corresponding ABC. The ABCs are not intended to be full-featured; instead, they specify the part of the API that will be guaranteed. For example, the union() method for built-in sets allows two or more set arguments, but the corresponding method for the ABC is limited to two arguments. This was an intentional difference, designed to make it easier for people to implement a Set class."
I suppose I should close this, unless there's some hope perhaps of some elaboration on that last line. Why couldn't collections provide the named methods too (in what way is this "easier"?)