title: How to modify large text documents systematically?
body: <p>I have a several hundred-thousand-line text document, and I need lines modified systematically. I need the file to be modified systematically so that it can be processed by another program.</p>
  
  <p>At the beginning of every line, I need to add a string, and after every line of text, I need to add 2 more lines with text in them.</p>
  
  <p>How would I go about doing this?</p>