title: What are Mixins (as a concept)
body: <p>I'm trying to get my head around the Mixin concept but I can't seem to understand what it is.
  The way I see it is that it's a way to expand the capabilities of a class by using inheritance.
  I've read that people refer to them as "abstract subclasses". Can anyone explain why?</p>
  
  <p>I'd appreciate if you'd explain your answer based on the following example (From one of my lecture slideshows):
  <img src="https://i.stack.imgur.com/uxgOo.png" alt="A C++ Mixin Example"></p>