title: When to use lambda, when to use Proc.new?
body: <p>In Ruby 1.8, there are subtle differences between proc/lambda on the one hand, and <code>Proc.new</code> on the other. </p>
  
  <ul>
  <li>What are those differences?</li>
  <li>Can you give guidelines on how to decide which one to choose?</li>
  <li>In Ruby 1.9, proc and lambda are different. What's the deal?</li>
  </ul>