title: RXJava how to wait/insert a subsequence
body: <p>I have a Sequence of Observable commands. The commands are executed sequential. Now one of these commands has to dynamically create a second Sequence of Observable commands when its executed. Before the first sequence continues with the next Observable, the second sequence has to be finished.
  This looks like a merge in git (0 represents an observable command):</p>
  
  <pre><code>Seq1:   -0-0-0-0         0-0-0-0-
                  \       /
  Seq2:            0-0-0-0
  </code></pre>
  
  <p>I use concatEager() to achieve sequential execution within one Sequence. The problem is that the current execution looks like following (=> after subscribing to seq2 it is executed in parallel to seq1).</p>
  
  <pre><code>Seq1:   -0-0-0-0-0-0-0-0-
                  \       
  Seq2:            0-0-0-0-
  </code></pre>
  
  <p>Is there a way to wait/block in the forth observable of seq1 until seq2 has finished? Or to merge after subscription of seq1?</p>