title: In TypeScript an interface can extend a class, what for?
body: <p>in <a href="https://www.typescriptlang.org/docs/handbook/classes.html" rel="noreferrer">TypeScript Handbook</a>
  in 'Using a class as an interface' section, there is an example of an interface which extends a class.</p>
  
  <p><code>class Point { ... }
  interface Point3d extends Point {...}</code></p>
  
  <p>When can this be useful? Do you have any practical examples of this?</p>