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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.
it sounds like the infamous mutable default arguments Python gotcha. But without more information it's impossible to be sure, and the OP is gone.
nice kata. however, i used a function with a default argument for my preorder, postorder and inorder functions, and the tests persist the value of this default parameter. I could write this program outside of the codewars interface, but can't in the way the interface expects me to write it. This seems to be a bug in the interface.