title: Merging two lines in a text file if they start with the same expression
body: <p>I have one file that looks like this</p>
  
  <pre><code>&gt;Unc14086 
  AGAGUUUGAU 
  &gt;Unc35443
  GCACGAGAAA
  </code></pre>
  
  <p>So, every n (n may vary) lines the next line starts with ">", that is the beginning of a new block of information.</p>
  
  <p>I have another tab-delimited file:</p>
  
  <pre><code>Unc14806 InformationalTextExample
  Unc35433 InformationalTextExampleII
  </code></pre>
  
  <p>My goal is to parse the second file with information found in lines starting with ">" in the first file. Whenever a matching pair occurs, i want to write "InformationalTextExample" in that line, possibly separated by "_":</p>
  
  <pre><code>&gt;Unc14086_InformationalTextExample
  AGAGUUUGAU 
  &gt;Unc35443_InformationalTextExampleII
  GCACGAGAAA 
  </code></pre>
  
  <p>How would that be possible?</p>
  
  <p>Thank you!</p>