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It appears to be a pretty common mistake to think that the argument of str.strip/lstrip/rstrip is a substring rather than a set of characters. To allow a more clearer notation, it would be nice if these functions also accepted an argument other than a string, for example a set or any collection. Then you could write, for example:
a.strip({'a', 'b', 'c'})
I suggest to either add support for sets specifically, or more generally anything that supports the "in" operator.
I can try to code it up if it sounds acceptable.