title: Linq to Xml and custom xml entities
body: <p>I want to create a MathML document from an expression tree using Linq to xml, but I cannot figure out how to use the MathML xml entities (such as &ApplyFunction; and &amp;InvisibleTimes):
  When I try to create directly a XElement using</p>
  
  <pre><code>XElement xe = new XElement("mo", "&amp;InvisibleTimes");
  </code></pre>
  
  <p>it justs escapes the ampersand (which is no good).
  I also tried to use XElement.Parse</p>
  
  <pre><code>XElement xe = new XElement.Parse("&lt;mo&gt;&amp;InvisibleTimes&lt;/mo&gt;");
  </code></pre>
  
  <p>but it fails with an System.XmlException: Reference to undeclared entity 'InvisibleTimes'
  How can I declare the entity or ignore the checks?</p>