1/5 "Ugly side of default interface methods" time. Interface with two methods, one with a default implementation. What will the following program print? 
2/5 If you guessed 21, you're right, but don't pop the champagne yet. Let's change "class Adder" to "struct Adder" - what will it print now?
3/5 Hint: it's not 21 anymore. The important piece of knowledge to crack this is knowing that interfaces are reference types. To call a default interface method, we need a reference type. For structs, this means boxing.
4/5 Here's annotated version of AddAndPlusPlus.
5/5 To make this print 21 again, we need to provide an implementation of PlusPlus on the struct directly. Default interface methods don't play well with structs.
Bonus nightmare: what will the first and second call to Horror return? (Don't bother re-typing this - it's a mental exercise. We didn't implement this part for .NET Core 3.0 because I had better things to do - this is going to throw a NotSupportedException for now.)
