title: How to merge two streams (without nulls) and apply conditions on pairs?
body: <p>Consider I have two streams of data, is there a way to merge them and apply conditions on data between these two streams? For example</p>
  
  <pre><code>Stream A : A, B, C, D....
  Stream B : -, A, -, -....
  Composed : (A,-),(B,A),(C,-),(D,-)....
  </code></pre>
  
  <p>How to get composed stream above using rxjs? I would like to apply conditions on composed streams to raise some notifications. Also would it be possible to use last known non-null data for example see the composed stream below.</p>
  
  <pre><code>Stream A : A, B, C, D....
  Stream B : 1, null, 2, null....
  Composed : (A,1),(B,1),(C,2),(D,2)....
  </code></pre>
  
  <p>I've just started playing with reactive streams idea, so please correct me if I've misunderstood the idea of reactive streams.</p>