title: Undefine variable in Ruby
body: <p>Let's say I'm using <code>irb</code>, and type <code>a = 5</code>. How do I remove the definition of <code>a</code> so that typing <code>a</code> returns a <code>NameError</code>?</p>
  
  <p>Some context: later I want to do this:</p>
  
  <pre><code>context = Proc.new{}.binding
  context.eval 'a = 5'
  context.eval 'undef a'  # though this doesn't work.
  </code></pre>