They should have just used `flat_map`, which does everything that `filter_map` does and more. In this case the call to `Result::ok` could have been omitted in f

They should have just used `flat_map`, which does everything that `filter_map` does and more. In this case the call to `Result::ok` could have been omitted in favour of the identity function (which strangely isn't in the standard library). I wonder why rust even has `filter_map`.