title: Is Rx.Subject a hot observable?
body: <p>The code</p>
  
  <pre><code>const a = new Rx.Subject().do(x =&gt; console.log('a'))
  const b = a.mapTo(0)
  const c = a.mapTo(1)
  const d = Rx.Observable.merge(b, c)
  d.subscribe(x =&gt; console.log('d'))
  a.next(3)
  </code></pre>
  
  <p>And the output</p>
  
  <pre><code>a
  d
  a
  d
  </code></pre>
  
  <p>Why does a got printed twice? Isn't Rx.Subject a hot observable?</p>