title: XSD Signature issue
body: <p>I cannot resolve this error about <code>&lt;xs:element ref="ds:Signature"/&gt;</code>.
  I need some help please.</p>
  
  <blockquote>
    <p>Copyright (C) Microsoft Corporation. All rights reserved. Schema
    validation warning: The '<a href="http://www.w3.org/2000/09/xmldsig#:Signature" rel="noreferrer">http://www.w3.org/2000/09/xmldsig#:Signature</a>'
    el ement is not declared. Line 162, position 8.</p>
    
    <p>Warning: Schema could not be validated. Class generation may fail or
    may produce  incorrect results.</p>
    
    <p>Warning: cannot generate classes because no top-level elements with
    complex type  were found.</p>
  </blockquote>
  
  <p><strong>XSD</strong></p>
  
  <pre><code>&lt;xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"  
              xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
    attributeFormDefault="unqualified" elementFormDefault="qualified"&gt;
  
    &lt;xs:import namespace="http://www.w3.org/2000/09/xmldsig#"
               schemaLocation="http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/xmldsig-core-schema.xsd"/&gt;
  
               &lt;xs:complexType name="SobreCheques"&gt;
          &lt;xs:annotation&gt;
              &lt;xs:documentation&gt;Definition of the ...&lt;/xs:documentation&gt;
          &lt;/xs:annotation&gt;
          &lt;xs:sequence&gt;
               ...
        &lt;xs:element ref="ds:Signature"/&gt;
          &lt;/xs:sequence&gt;
      &lt;/xs:complexType&gt;
  &lt;/xs:schema&gt;
  </code></pre>