title: "invalid (do_set) left-hand side to assignment" when trying to summarize
body: <p>I'm trying to summarize some grouped data and have the following code:</p>
  
  <pre><code>yearly.users &lt;- group_by(df, Year)
  
  yearly.users &lt;- summarise(yearly.users,
                           total.users &lt;- sum(participants),
                           mean.users &lt;- mean(participants))
  </code></pre>
  
  <p>This actually works the first time I pass through it. But if I try to change the parameters in the summarize variable, I get:</p>
  
  <pre><code>Error in summarise_impl(.data, dots) : 
    invalid (do_set) left-hand side to assignment
  </code></pre>
  
  <p>Don't know how to interpret this. Any help is appreciated.</p>