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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.
The "Incorrect id number format" part of the description is not exhaustive.
The description doesn't explain what to do about x.
It is not clear what a1 equals to when the last character is x.
The description probably lacks a formula to calculate x.
It is impossible to calculate summation of [ a18 * W18, a17 * W17, . . . , a1 * W1 ] without knowing a1.