Returns either the closest index in the index set that is less than a specific index or the not-found indicator.
- iOS 2.0+
- macOS 10.3+
- UIKit for Mac 13.0+Beta
- tvOS 9.0+
- watchOS 2.0+
Index being inquired about.
Closest index in the index set less than
index; NSNotFound when the index set contains no qualifying index.