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Arc seams to be just like Scheme. I can't see anything that makes it mutch better. What is better in Arc then in Scheme?

What is the concurency story (does it have one)? Does arc have full tail call optimisation?

Does arc have full tail call optimisation?

Yes. Arc's reference implementation inherits TCO from Racket.

pg wrote it and the backend of the site is written in it. You can read more about it here: http://www.paulgraham.com/arc.html