I was confused because the programming language that Hacker News is written in is also called Arc and I was like "Wait, did PG turn over maintenance to an independent foundation? Are they giving up now? What does this mean for HN?"

I made two additional wrong guesses, the Advanced RISC Computer class of machine firmware and the file archiving utility from System Enhancement Associates. (-:

(Back when the headline was "Arc is Sunsetting". I am so 2 minutes ago.)