title: Tracker.autorun behaviour on multiple and nested reactive vars
body: <p>I could not find detailed info on on the behaviour of <code>Tracker.autorun</code> in Meteor.</p>
  
  <p>E.g. inside of the autorun function what happens (and why):</p>
  
  <ul>
  <li>if I get the same reactive var twice</li>
  <li>if I get Meteor.user() and Meteor.userId()</li>
  <li>what about reactive vars got in functions nested inside the autorun function</li>
  <li>what about multiple different reactive vars</li>
  </ul>