After a long time I haven’t touched Ruby I want to come back. I want to
refresh my memory about the various methods that come with the built-in
classes; e.g., String, Array, Hash. So I do, for example, “ri String”
and here’s a problem: I get a long list of methods but some of them are
defined by some gem or 3rd party library. That’s a problem, because I
want to see only the methods defined by Ruby itself. How do I do this?