title: how to insert row in first line of text file?
body: <p>I have a test file that contains</p>
  
  <pre><code>1,2,3
  2,3,4
  5,6,7
  </code></pre>
  
  <p>I want to insert this into the first line: <code>A,B,C</code></p>
  
  <p>So that I get:</p>
  
  <pre><code>A,B,C
  1,2,3
  2,3,4
  5,6,7
  </code></pre>
  
  <p>How can I do this?</p>