Is IObservable available in unity?

Its supposed to be in the systems namespace but it doesnt show up? Is this available for unity?

http://docs.unity3d.com/410/Documentation/ScriptReference/MonoCompatibility.html

Or try setting your Player Settings to .NET 2.0 rather than .NET 2.0 Subset

With 2017 beta Runtime .NET 4.6 I think we should have access to System.IObservable, but I am getting assembly reference errors when attempting to use.

The type System.IObservable1’ is defined in an assembly that is not referenced. Consider adding a reference to assembly `System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a’