title: dplyr comma (,) syntax appears broken
body: <pre><code># Code chunk 1
  library(tidyverse)
  df &lt;- tibble(a = c(NA, 1, 2), b = c(5, 5, 5))
  </code></pre>
  
  <p>Please create the data frame above. If you then run this code chunk 2</p>
  
  <pre><code># Code chunk 2 
  df %&gt;%
     mutate(
       c = abs(lag(a, 1) - a),
       d = sum(.$c, na.rm = TRUE) / (length(.$c) - 1)
     )
  </code></pre>
  
  <p>you get this error <em>"Unknown or uninitialised column: 'c'."</em>. Yet I can run the same code, arranged differently, and get what I desire (no errors, proper output).</p>
  
  <pre><code># Code chunk 3
  df %&gt;%
    mutate(c = abs(lag(a, 1) - a)) %&gt;%
    mutate(d = sum(.$c, na.rm = TRUE) / (length(.$c) - 1))
  </code></pre>
  
  <p>Why doesn't the second code chunk work? I don't want to have to string together a series of <code>mutate(x) %&gt;% mutate(y) %&gt;% mutate(z) %&gt;% etc.</code> when a simple <code>mutate(x, y, z)</code> is much tidier.</p>