Can't find Substitute class and Return method

I was following this tutorial. At about 10:30 in the video, they used the Substitute class and Return function. I haven’t been able to find either of those things in the documentation. Does anyone know how to fix this?

It’s part of a stand-in library that lets you test things when you don’t have all the other stuff you’re not testing (but need for the test) ready. Check it out at the github site, and you might like this video too.