title: SAS transpose wide format to long format
body: <p>I have a SAS dataset that I need to transpose from wide format to long format
  data that I have:</p>
  
  <pre><code>DATES  Year1   Year2   Year3
  Jan      100    200     300
  </code></pre>
  
  <p>Data I want:</p>
  
  <pre><code>DATES  Year  Income
  Jan     1      100
  Jan     2       200
  Jan     3       300
  </code></pre>