title: schema validate xml containing boolean values
body: <p>Hi I have a xml like this:</p>
  
  <pre><code>&lt;xml&gt;&lt;fullname&gt;
  &lt;name attrib="true"/&gt;
  &lt;lastname1 attrib="false"/&gt;
  &lt;lastname2 attrib="false"/&gt;&lt;/fullname&gt;&lt;/xml&gt;
  </code></pre>
  
  <p>I need to create a schema to validate it in such a way that it will allow only 1 attrib with value "true" (and the rest of them must be false [the attrib attribute is defined to be xs:boolean]), so i added a unique check in the element fullname like this:</p>
  
  <pre><code>   &lt;xs:unique name="attribcheck"&gt;
  &lt;xs:selector xpath="name|lastname1|lastname2"/&gt;
  &lt;xs:field xpath="@attrib"/&gt;
  </code></pre>
  
  <p></p>
  
  <p>Of course, it will detect that there is a duplicated "true", but also, it will detect a duplicated "false".
  Does anyone know if there is a way to set a restriction on which value to apply the unique constraint? meaning that i can ensure that only one of them is "true" at any given time</p>