title: How costly is 'extend' in ruby?
body: <p>First of all, I know how <code>extend</code> and <code>include</code> work, and what they're usually used for etc. Whether it is a good idea or not is not part of my question.</p>
  
  <p>My question is: how expensive is <code>extend</code>? It's a common Javascript technique to extend instances and singleton objects. One could do something similar in Ruby, but would it be slow if used on a lot of objects?</p>