title: Julia: check whether array entry is undef
body: <p>What is the best way in Julia to check whether an array entry is #undef?</p>
  
  <p>Example:</p>
  
  <pre><code>julia&gt; a = Array(Vector,2)
  julia&gt; isdefined(a[1])  # fails
  julia&gt; isempty(a[1])    # fails
  </code></pre>