title: What is this smiley-with-beard expression: "<:]{%>"?
body: <p>I came across the following program, which compiles without errors or even warnings:</p>
  
  <pre><code>int main(){
    &lt;:]{%&gt;; // smile!
  }
  </code></pre>
  
  <p><a href="http://ideone.com/inXuVc">Live example.</a></p>
  
  <p>What does the program do, and what is that smiley-expression?</p>