This makes it hard to use lists as keys for tables:
arc> (let x (table)
(= (x '(a b)) t)
(x (list 'a 'b)))
As list returns a Scheme list and the key is not ac-niltree'd before being passed to hash-table-get or hash-table-put!, the two lists are not Scheme hash table equal? and so represent different keys in the table.
Ok, this is now fixed. The Arc def of list now makes an explicit copy. Rtm has ambitions of one day making rest args ac-niltreed copies in ac.scm, whereupon we can return to the nice simple def of list.