title: How to concatenate xsl:value-of with separator
body: <p>Given XML:</p>
  
  <pre><code>&lt;issueDate day="30" month="09" year="2015"/&gt;
  </code></pre>
  
  <p>I wanted to build a string that output: 2015-09-30</p>
  
  <p>This worked:</p>
  
  <pre><code>&lt;xsl:variable name="issueDate" as="xs:string"&gt;
      &lt;xsl:value-of select="concat(//issueDate/@year,'-',//issueDate/@month,'-',//issueDate/@day)" /&gt;
  &lt;/xsl:variable&gt;
  </code></pre>
  
  <p>...</p>
  
  <pre><code>&lt;xsl:value-of select="$issueDate"/&gt;
  </code></pre>
  
  <p>But this threw an error (expected EOF, found ','):</p>
  
  <pre><code>&lt;xsl:value-of select="//issueDate/@year,//issueDate/@month,//issueDate/@day" separator="-" /&gt;
  </code></pre>
  
  <p>What is the syntax to select more than one value using separator attribute?</p>