sref is "set reference". It's used for indexed assignment.
arc> (= xs '(a b c)) (a b c) arc> (sref xs 'd 0) d arc> xs (d b c)
arc> (sig 'map1) (f xs) arc> (def f (a b c) (+ a b c)) #<procedure: f> arc> (sig 'f) (a b c)