title: Update formula objects in R with offset function
body: <p>I have an R formula object:</p>
  
  <pre><code>  R&gt; formula.obj &lt;- Y ~ 1 + X + offset(Z)
  </code></pre>
  
  <p>I want to get rid of offset(Z) and obtain:</p>
  
  <pre><code>  R&gt; formula.obj.want &lt;- Y ~ 1 + X
  </code></pre>
  
  <p>It seems that update function does NOT work in this scenario:</p>
  
  <pre><code>  R&gt; update(formula.obj,.~.-offset(Z))
    Y ~ X + offset(Z)
  </code></pre>
  
  <p>Is there way to get formula.obj.want from formula.obj?</p>