nth

(nth coll i) / (nth coll i default)

Answers the ith element of coll: a vector or string indexes directly, a list or a seq steps through it one element at a time, so a lazy input realizes only up to i and an infinite one answers. Past either end the 3-arity answers default and the 2-arity nil. A map, a set, a record and anything that is no collection signal UnsupportedOperationException, like the oracle; a vector pattern destructures through nth, so it refuses them too.

Deviation: nth past the end answers the default (nil without one), where the oracle throws. As a VALUE nth is a (collection index) lambda -- the Clojure order -- since a bare underlying nth takes the index first.