title: What is wrong with XPath expression
body: <p>Sorry I`m new for this stuff.
  I have this (was unable to format my code properly so I splitted it in two blocks bellow):</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;bulkCmConfigDataFile xmlns:es="EricssonSpecificAttributes.14.04.xsd"
     xmlns:un="utranNrm.xsd" xmlns:xn="genericNrm.xsd"
     xmlns:gn="geranNrm.xsd" xmlns="configData.xsd"&gt;
     &lt;fileHeader fileFormatVersion="32.615 V4.5" vendorName="Ericsson"/&gt;
     &lt;un:UtranCell id="U1"&gt;
        &lt;un:attributes&gt;
           &lt;un:localCellId&gt;1&lt;/un:localCellId&gt;
           &lt;un:uarfcnUl&gt;9886&lt;/un:uarfcnUl&gt;
           &lt;un:uarfcnDl&gt;10836&lt;/un:uarfcnDl&gt;
           &lt;un:primaryScramblingCode&gt;335&lt;/un:primaryScramblingCode&gt;
           &lt;un:primaryCpichPower&gt;300&lt;/un:primaryCpichPower&gt;
           &lt;un:maximumTransmissionPower&gt;400&lt;/un:maximumTransmissionPower&gt;
           &lt;un:primarySchPower&gt;-18&lt;/un:primarySchPower&gt;
           &lt;un:cId&gt;1&lt;/un:cId&gt;
           &lt;un:userLabel&gt;U_TST33_1&lt;/un:userLabel&gt;
           &lt;un:secondarySchPower&gt;-35&lt;/un:secondarySchPower&gt;
           &lt;un:bchPower&gt;-31&lt;/un:bchPower&gt;
           &lt;un:lac&gt;65006&lt;/un:lac&gt;
           &lt;un:rac&gt;6&lt;/un:rac&gt;
           &lt;un:sac&gt;1&lt;/un:sac&gt;
           &lt;un:uraList&gt;65006&lt;/un:uraList&gt;
        &lt;/un:attributes&gt;
     &lt;/un:UtranCell&gt;
     &lt;fileFooter dateTime="2016-04-13T15:11:31Z"/&gt;
  &lt;/bulkCmConfigDataFile&gt;
  </code></pre>
  
  <p>I`m trying to do </p>
  
  <pre><code>xpath_expr = "//{configData.xsd}bulkCmConfigDataFile/{utranNrm.xsd}UtranCell"
  ucells = tree.xpath(xpath_expr)
  </code></pre>
  
  <p>But get en error:</p>
  
  <pre><code>File "lxml.etree.pyx", line 2186, in lxml.etree._ElementTree.xpath (src\lxml\lxml.etree.c:60010)
  File "xpath.pxi", line 359, in lxml.etree.XPathDocumentEvaluator.__call__ (src\lxml\lxml.etree.c:152734)
  File "xpath.pxi", line 227, in lxml.etree._XPathEvaluatorBase._handle_result (src\lxml\lxml.etree.c:151097)
  File "xpath.pxi", line 213, in lxml.etree._XPathEvaluatorBase._raise_eval_error (src\lxml\lxml.etree.c:150950)
  </code></pre>
  
  <p>lxml.etree.XPathEvalError: Invalid expression</p>
  
  <p>What's wrong with what I am doing?
  By the way, executing this</p>
  
  <pre><code>tree = etree.parse(infile)
  root = tree.getroot()
  print 'Root tag: ', root.tag
  for child_of_root in root:
          print 'Child tags: ', child_of_root.tag, child_of_root.attrib
  </code></pre>
  
  <p>Gives this output: </p>
  
  <pre><code> - Root tag:  {configData.xsd}bulkCmConfigDataFile
   - Child tags:  {utranNrm.xsd}UtranCell {'id': 'U1'}
  </code></pre>