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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.
In short( Extremly short) the [:size] is slicing it until the number you put there so if it's 6, the output is up to index 6 but not included (remember the index in python starts at 0)
so it '10' * 6 and it omitts the 6th index.
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It's Python list comprehension. Make a research about it if you don't know it. For the rest, take a pencil and a paper with a small example and try to figure out what the code does.