Fix Prev() to behave like Next() in reverse
Enumerator.Prev() would return the next item if there was no hit. This fix instead changes Prev to return previous item as expected. Resolving issue https://github.com/cznic/b/issues/6.
Enumerator.Prev() would return the next item if there was no hit. This fix instead changes Prev to return previous item as expected. Resolving issue https://github.com/cznic/b/issues/6.