Xamarin.iOS cannot find type BaseType attribute

I was looking into Objective-C protocols and delegates and was trying to implement a custom one in Xamarin.iOS. I was following the example in the Xamarin documentation for "Binding Protocols" ``` [BaseType (typeof(NSObject))] [Model][Protocol] public interface INITableViewCellDelegate { } ``` for some reason I cannot find the BaseType attribute. I cannot reference it. My current list of namespaces for the project are: ``` using System; using System.Drawing; using MonoTouch.Foundation; using MonoTouch.UIKit; using MonoTouch.CoreGraphics; using MonoTouch.ObjCRuntime; ``` I'm expecting that it should be under System or MonoTouch.ObjCRuntime. Can anyone shed some light? I've checked other questions on StackOverflow involving binding protocols and many examples show the same MonoTouch usings. Thank you