Added lru-use function

scribblings/lru-cache.scrbl

@@ -74,6 +74,15 @@ If capacity is reached, the least recently used item is removed.
 Returns @racket[l].
 }
 
+@defproc[(lry-use [l lru?] [key any/c] [default any/c]) any/c]{
+Finds the given @racket[item] by @racket[key] (this must be accomodated
+in the compare function (see @tt{#:cmp}). When the item is found
+using the key, it will not only return the item, but also move the
+item to the front of the cache and update the expiry parameter.
+
+When the item is not found, it will return @racket{default}. 
+}
+
 @defproc[(lru-has? [l lru?] [item any/c]) boolean?]{
 Returns @racket[#t] if @racket[item] is currently present (and not expired),
 otherwise @racket[#f]. This call also performs lazy cleanup of expired items.