title: svgwrite - How to enable write/add other objects as layers of an existing .svg file
body: <p>I have an svg file and I want to insert other svg object inside this existent.</p>
  
  <pre><code>import svgwrite
  
  dwg = svgwrite.Drawing('model.svg')
  square = dwg.add(dwg.rect(20,20),(80,80), fill='blue'))
  dwg.save()
  </code></pre>
  
  <p>it returns a new file with this shape ignoring my previous file.
  How could I write this?</p>
  
  <p>Thank's</p>