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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 comment is hidden because it contains spoiler information about the solution
Indeed many solutions directly filter out string values and leave everything else in the list, but as @Chrono79 said the Kata input only consists in intgeres and strings.
Furthermore, there exist realistic cases in which one can be interested in retaining only integers or filtering out only specific data types.