title: Pipe with exposition of variables magrittr
body: <p>I am trying to understand how the magrittr exposition operator <code>%$%</code> works.
  I read that the usage should be similar to the <code>with</code> function, however it doesn't behave as expected.
  I don't understand why in this example it returns an error</p>
  
  <pre><code>&gt; with(iris, sum(Sepal.Length)/sum(Sepal.Width))
  [1] 1.911252
  &gt; iris %$% sum(Sepal.Length) / sum(Sepal.Width)
  Error: object 'Sepal.Width' not found
  </code></pre>
  
  <p>Thank you for your help</p>