title: What is purpose of dot before variables (i.e. "variables") in the R Plyr package?
body: <p>What is purpose of dot before variables (i.e. "variables") in the R Plyr package?</p>
  
  <p>for instance, from the R help file: </p>
  
  <pre><code>ddply(.data, .variables, .fun = NULL, ...,
      .progress = "none", .drop = TRUE, .parallel = FALSE)
  </code></pre>
  
  <p>Any assistance would be greatly appreciated</p>