In the description, consider calling it an array instead of a list. And consider changing it a little bit so that they can't just call index on the block. I don't know, I feel like it can be just a tiny bit more challenging without being too hard.

I agree with cmpolis. If the purpose of this is to learn monkey patching, then alter it slightly so they cannot just use alias.

Maybe rephrase: "Learn to use Ruby's feature to re-open class to add new functionalities to existing classes and objects."

to:

"Ruby lets you re-open classes so you can add new functionality to existing classes and objects."

It's a good kata. I had to read the description several times before I got it.

Maybe "you must be able to subscribe n number of blocks to the same key" could be "you must be able to subscribe multiple blocks to the same key"

Think about it this way... if you look at the diagrams in the kata, it is a spiral. Everytime it goes from left to right, it is on the top most row that hasn't been processed, every time it goes from up to down, it's on the right-most column that hasn't been processed, when it's right to left, it's always the very bottom and when it's down to up it's always the left-most column. So if you think about it as 4 different phases that you have to do over and over again, making sure not to process any element twice, then maybe it's more manageable. That's how I thought of the solution when I finally figured it out. There are other more graceful solutions though, and many other ways of solving it of course.

it also doesn't support "http://subdomain.domain.com". my solution doesn't either, but I was trying to write the minimal amount to pass the tests ;) because lazy