what about things like __*** ?

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Hi, all~

when reading the tutorial I met some trouble.(Maybe trifle to you all
but really trouble to me)

That is I couldnt figure out what __*** is. Here is the description of
that article but even confused me:

There is limited support for class-private identifiers. Any identifier
of the form __spam (at least two leading underscores, at most one
trailing underscore) is now textually replaced with _classname__spam,
where classname is the current class name with leading underscore(s)
stripped.

What's it trying to say please ???