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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.
Use a copy (read the Ruby docs about how to make one) so your function doesn't mutate it and the expected result is right.
The control function use the input values your function left, so after you removed the chars it returns false instead of true.
Strings are mutable in Ruby, if your code fails the random tests I'm almost sure, you're mutating the input values.