title: aiohttp: how to add sub_app to sub_app?
body: <p>I need to add sub_app to sub_app. Here is my code</p>
  
  <pre><code>app = web.Application()
  
  subapp = web.Application()
  subapp.router.add_get('/page1/', handler)
  app.add_subapp('/wt/', subapp)
  
  subsubapp = web.Application()
  subsubapp.router.add_get('/page2/', handler)
  subapp.add_subapp('/sub/', subsubapp)
  </code></pre>
  
  <p>Why the page <code>/wt/sub/page2/</code> is not found?
  I think that the path for <code>subsubapp</code> is a concatenation of <code>/wt/</code> (<code>sub_app</code>) and <code>/sub/</code> (<code>subsubapp</code>). Am I wrong? What is the rule?</p>
  
  <p>Regards,
  Valery. </p>