Loading collection data...
Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
I worked out the same solution. Can I ask why you have the 'trim' at the end? Will the final 'join' ever produce an outer newline character?
What specifically did you find unclear?
Could be a little clearer in the description with regards to how it should throw an error, although the code snippet near the bottom makes up for this. I like it mostly. :)
Impressive solution!