title: Unique element in XSD sequence
body: <p>I'd like to have an XSD to valide an XML containing file elements with many aliases but each alias with a different value. </p>
  
  <p>This is my XML:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8" standalone="yes" ?&gt;
  &lt;folder&gt;
    &lt;files&gt;
      &lt;fileList&gt;
        &lt;file&gt;
          &lt;title&gt;Document1&lt;/title&gt;
          &lt;fileAlias&gt;
            &lt;alias&gt;Alias1&lt;/alias&gt;
            &lt;alias&gt;Alias2&lt;/alias&gt;
          &lt;/fileAlias&gt;
        &lt;/file&gt;
      &lt;/fileList&gt;
    &lt;/files&gt;
  &lt;/folder&gt;
  </code></pre>
  
  <p>The former XML is good because Alias1 != Alias2</p>
  
  <p>How could I specify it in my XSD:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;xs:schema attributeFormDefault="unqualified" elementFormDefault="qualified" version="1.0"
             xmlns:xs="http://www.w3.org/2001/XMLSchema"&gt;
    &lt;xs:element name="folder" type="folderType" /&gt;
    &lt;xs:complexType name="folderType"&gt;
      &lt;xs:sequence&gt;
        &lt;xs:element name="files" type="filesType" /&gt;
      &lt;/xs:sequence&gt;
    &lt;/xs:complexType&gt;
    &lt;xs:complexType name="filesType"&gt;
      &lt;xs:sequence&gt;
        &lt;xs:element name="fileList" type="fileListType" /&gt;
      &lt;/xs:sequence&gt;
    &lt;/xs:complexType&gt;
    &lt;xs:complexType name="fileListType"&gt;
      &lt;xs:sequence&gt;
        &lt;xs:element maxOccurs="unbounded" name="file" type="fileType" /&gt;
      &lt;/xs:sequence&gt;
    &lt;/xs:complexType&gt;
    &lt;xs:complexType name="fileType"&gt;
      &lt;xs:sequence&gt;
        &lt;xs:element name="title" type="xs:string" /&gt;
        &lt;xs:element name="fileAlias" type="fileAliasType" minOccurs="0" maxOccurs="unbounded" /&gt;
      &lt;/xs:sequence&gt;
      &lt;xs:attribute name="id" type="xs:string" /&gt;
      &lt;xs:attribute name="type" type="xs:int" /&gt;
    &lt;/xs:complexType&gt;
    &lt;xs:complexType name="fileAliasType"&gt;
      &lt;xs:sequence&gt;
        &lt;xs:element maxOccurs="unbounded" name="alias" type="xs:string" minOccurs="1" /&gt;
      &lt;/xs:sequence&gt;
    &lt;/xs:complexType&gt;
  &lt;/xs:schema&gt;
  </code></pre>
  
  <p>I've tried with <code>xsd:unique</code> but I think is for attributes not for XML element text values.</p>