title: Exists function in q/kdb
body: <p>I need to write a function in q/kdb which takes a variable v and returns 1b if v is
  defined and 0b if it is not:</p>
  
  <pre><code>$ a:2
  $ doesExist`a
  1b
  $ doesExist`b
  0b
  </code></pre>
  
  <p>Any ideas appreciated.</p>