title: Combining an observable with the latest value of another observable
body: <p>I have 2 observables <code>A</code> and <code>B</code>.</p>
  
  <p>I would like to generate <code>C</code> that <strong>emits only when A emits</strong>, with a combination of the value just pushed in <code>A</code>, and the most recent value in <code>B</code>.</p>