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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.
This is just for simplification, so as not to rewrite the alphabet.
index() raise ValueError when the substring is not found, and it could be not found, because in the description "If anything in the text isn't a letter, ignore it and don't return it." So, not letter could be in text, but you loking for a letter only.