Well built, but I think there are definitely some ways you could've incorporated the optional theme to make it a little more interesting. Maybe a winter themed slot machine or something like that
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Yes you are absolutely right. My idea was to give players the ability to freeze a reel after each round. This turned out to be unintuitive. If you freeze a 7 you actually do not improve your average payout (because now you can only win with two more sevens) and the dominant strategy would be to freeze a cherry each round. This kinda felt boring to me so I left it out.
And yes a winter themed machine would have worked as well ;)