title: Referencing a dataframe recursively
body: <p>Is there a way to have a dataframe refer to itself?</p>
  
  <p>I find myself spending a lot of time writing things like <code>y$Category1[is.na(y$Category1)]&lt;-NULL</code> which are hard to read and feel like a lot of slow repetitive typing. I wondered if there was something along the lines of:</p>
  
  <p><code>y$Category1[is.na(self)] &lt;- NULL</code> I could use instead.</p>
  
  <p>Thanks</p>