@sethvargo `#pretty_print` is the interface for the [stdlib `PP` class](http://www.ruby-doc.org/stdlib-2.0/libdoc/pp/rdoc/PP.html). If you attempt to call `pp` on an object and it defines `#pretty_print`, `PP` will use that method instead of using it's own printing method, and pass itself in as the first argument.

I honestly thought that's what this method was for, just wasn't sure why I was getting `wrong number of arguments (1 for 0)` errors when ran into failing specs (RSpec must use `pp` to display diffs in errors?). Should I link to the stdlib in the method docs?