The letter 'z' was not a possible letter in mutation #16
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First of all thank you for the code that you have provided, it saved me a lot of time!
I encountered this bug while messing around the java code. I tried to use the algorithm with a string that contained 'z' and iterating, because it was not possible for 'z' to occur in the mutation.
I fixed this in the java code and in C#, clojure, python, ruby and scala.