title: xtab error undefined columns selected
body: <pre><code>x = data.frame(a=c(T,F))
  x['foo'] = c(T,F)
  lm(foo ~ ., data = x)  # works
  lm(. ~ foo, data = x)  # fails
  xtab(. ~ foo, data = x)  # fails
  </code></pre>
  
  <p>The last line is the one I really want but it throws an "undefined columns selected" error. I need to set the dimension of <code>x</code> how I'm doing it because the name of the dimension comes from a variable.</p>
  
  <p>I've been messing with this for a while now and can't figure it out. What am I doing wrong?</p>