title: how to insert a SAML Assertion to SOAPHeader
body: <p>NB-6.8 jdk-1.6.14 WSS4j-1.5.8 ,OpenSAML-2.3.0
  I try to modify WSS4J-1.5.8 to operate SAML 2.0, and I could not insert a generated assertion to existing SOAP Header, the code like this:</p>
  
  <pre><code>Assertion assertion = createAssertion("some subject", "some issuer");
  
      Document doc = docBuilder.parse("request.xml");
  
      Element parent = doc.getDocumentElement();
  
      Node node = parent.getElementsByTagName("soapenv:Header");
      node.appendChild(assertion.getDOM());
  </code></pre>
  
  <p>Error  : WRONG_DOCUMENT_ERR: A node is used in a different document than the one that created it.</p>
  
  <p>but in wss4j, some functions like insertSecurityHeader do so(appendChild or inserBefore) ,too. They do work, why it's wrong in my code? what I miss.</p>