title: How to add property to external namespaced interface provided by lib
body: <p>I'm having trouble adding a property to an interface provided by the <code>@feathersjs/feathers</code> lib.</p>
  
  <p>This is their interface (I'll summarize and show the relevant parts)</p>
  
  <pre><code>// @feathersjs/feathers/index.d.ts
  
  declare const createApplication: Feathers;
  export = createApplication;
  
  interface Feathers {
      &lt;T = any&gt;(): createApplication.Application&lt;T&gt;;
      readonly ACTIVATE_HOOKS: unique symbol;
      version: string;
      default: Feathers;
  }
  
  declare namespace createApplication {
      //.... A BUNCH OF INTERFACES
      interface HookContext&lt;T = any&gt; {
          readonly app: Application;    // &lt;-- what I want to extend
          //.... A BUNCH OF PROPERTIES
      }
      //.... A BUNCH OF INTERFACES
      interface Application&lt;ServiceTypes = {}&gt; extends EventEmitter {
          //.... A BUNCH OF PROPERTIES
          // &lt;---- I WANT TO ADD A PROPERTY `logger?: any`
      }
  }
  </code></pre>
  
  <p>How do I accomplish this? (adding <code>logger?: any</code> as specified in the code block above)</p>
  
  <p>I've looked into declaration merging as a method because it looks like a really clean solution that preserves separation of concern, but I can't seem to get it working.</p>
  
  <p>Thanks for your help</p>