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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.
As noted already, the instructions are very unclear: upon simply reading, it is impossible to know how one "derives" letters from number.
There's a slight hint at step 4, but frankly this "derive" operation should be described as what it is: obtaining the ASCII sequence matching the number sequence.
The screenshot is not appearing for me :/
In Typescript, given the signature, you "can't" expect to have a1 or a2 being null, since the signature doesn't allow it. It's weird to have to handle this edge case in a typed language.