[stouset]

It's almost as if you didn't even bother reading the sentence immediately following.

If you're writing a library doing things like this is a bad idea (except in very limited circumstances). And to what seems to be every non-Rubyist's surprise, virtually no libraries in practice actually do this sort of thing so there are never problems in practice.

The times I do this sort of thing in my own application code, I put the methods in a module and have a hook that raises an exception if the methods already exist when the module is included. This is the best of both worlds: I can add features that don't exist, but am informed immediately when my app launches if such a method has already been defined.