As of now, I believe that non-alphabet characters can be input, and though you will likely get neither a bull or cow for these characters, it is slightly misleading that the feedback to the user doesn’t point this out. I know, it’s inherent to us that ‘3’ or ‘-’ is nonsense input, but it’s worth checking that only letters are allowed in a valid guess, for the sake of being thorough 
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The implementation for IsLowercase should catch these things as well.
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