title: Feed an observable with two more observables
body: <p>I have a scenario with 3 webservices, say A, B and C.</p>
  <p>A and B can be invoked without arguments. C requires parameters deriving from both A and B. For example:</p>
  <pre><code>   http.get(&quot;A&quot;).subscribe(x =&gt; console.log(x))
     // prints 'goofy'
     http.get(&quot;B&quot;).subscribe(y =&gt; console.log(y))
     // prints 'mickey'
     http.get(&quot;C?param1=goofy&amp;param2=mickey&quot;).subscribe(z =&gt; console.log(z))
     // prints 'daisy'
  </code></pre>
  <p>I wonder how to create a single Observable that does the job, and whose subscription return the result from webservice C.</p>
  <pre><code>  obs1 = http.get(&quot;A&quot;)
    obs2 = http.get(&quot;B&quot;)
    f3 = (x,y =&gt; http.get(&quot;C?param1=&quot;+x+&quot;&amp;param2=&quot;+y))
    bigObservable = someconcatenationof(obs1, obs2, f3)
  </code></pre>
  <p>and I would like that</p>
  <pre><code>  bigObservable.subscribe(z =&gt; console.log(z))
    // should print &quot;daisy&quot;
  </code></pre>
  <p>I suppose that this question can be a little bit generalized: A, B, C do not need to be webservices, they can just be some kind of Observable's, where C's constructor requires results from both A and B.</p>