Try using S7 instead of S3

[hadley]

This looks like a pretty simple conversion because purrr only defines a couple of new S3 classes via new_rate().

To tackle this task for TDD, you'll need some prior knowledge of S7. (But it's probably simple enough that you could pick up enough in one day to be effective, and if you try and don't succeed that would still be useful because it would indicate that the S7 documentation needs improvement).