I do not recommend learning it. If you want a Lisp to learn, Racket is a good choice. Arc is a terrible, poorly documented, poorly supported, unreadable language, whose primary goal is to save characters at the cost of comprehension. Which is lunacy. Characters are in far greater supply than comprehension.
moe, ARC is a dialect of Lisp. steve is correct that the original incarnation of Hubski was built on the bones of an HN clone. Though mk had cobbled together his own little monster that rob05c has uncobbled and is recobbling in a more amenable language.