I learned this method only because of this kata, and reading a Ruby book. I remembered this kata when I was looking around the array cap methods. It was useful to me, that I know.

In my opinion it's a great way to learn the Ruby Core API as I'm new to Ruby.

For the first some katas I coded in Java style, but after I saw some really good and compact solutions I started to read more carefully the Ruby doc :)

I would've thought the whole point was to avoid using the existing flatten method.
Guess anything goes, as long as it works and is within the constraints.