title: How to save an etree xml object? saving loading lxml etree objects issue
body: <p>I have a etree object called projectxml: </p>
  
  <pre><code>projetxml type &lt;type 'lxml.etree._Element'&gt;
  </code></pre>
  
  <p>I need to save it on disk, so I convert it to element tree:</p>
  
  <pre><code>savedxml=et.ElementTree(projetxml)
  savedxml.write('/home/simon/Vysis.xml')
  </code></pre>
  
  <p>An other script had to load the the Vysis.xml and two other files of the same kind:</p>
  
  <pre><code>vysis=et.parse('/home/simon/Vysis.xml')
  asi=et.parse('/home/simon/ASI.xml')
  psi=et.parse('/home/simon/PSI.xml')
  </code></pre>
  
  <p>Now asi, psi and vysis lxml objects are of the type for example:</p>
  
  <pre><code>&lt;lxml.etree._ElementTree object at 0xa7eaf8c&gt;
  </code></pre>
  
  <p>My problem is that I can no more do:</p>
  
  <pre><code>R=et.Element('DataBase')
  R.append(asi)
  R.append(psi)
  R.append(vysis)
  </code></pre>
  
  <p>because of the error:</p>
  
  <pre><code>R.append(asi)
    File "lxml.etree.pyx", line 697, in lxml.etree._Element.append (src/lxml  /lxml.etree.c:35471)
  TypeError: Argument 'element' has incorrect type (expected lxml.etree._Element, got lxml.etree._ElementTree)
  </code></pre>
  
  <p>I suppose I have two solutions. The first one could be to avoid to convert etree.Element to etree.ElementTree and to save it "directly", but I don't know how. The second solution would be to back convert etree.ElementTree to etree.Element type...There should be a clean solution to save/load a xml object?</p>