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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.
Still I would say that confusing stack trace is an issue with tests.
The last line of your error message tells you the most. Print your inputs to see what you're dealing with there, probably an empty container... Do you know about the
initial valuereferrred to about the reduce function?