Nu should be feature-complete, which means it implements everything that Arc 3.1 does.
However, seeing as how I've only spent the past 4-5 days working on it, obviously it may have bugs, might be slow, and is missing some new additions I want to add to it.
The "Details" section of the README should cover the stuff you want to know, but in case it doesn't, feel free to ask me.
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Thank you to awwx for writing ar, which was both an inspiration and the physical basis for large parts of Nu.