title: What does it mean for a data structure to be "intrusive"?
body: <p>I've seen the term <em>intrusive</em> used to describe data structures like lists and stacks, but what does it mean?</p>
  
  <p>Can you give a code example of an intrusive data structure, and how it differs from a non-intrusive one?</p>
  
  <p>Also, why make it intrusive (or, non-intrusive)? What are the benefits? What are the disadvantages? </p>