title: exclusions with '-' when using string versions (underscore suffix such as gather_()) of dplyr/tidyr functions
body: <p>Normally with dplyr/tidyr, I can achieve exclusions with negation</p>
  
  <pre><code>... %&gt;% gather(x, -y)
  </code></pre>
  
  <p>However, currently, I want some variables to be specified programmatically <em>and</em> be an exclusion, so ideally</p>
  
  <p>... %>% gather_(xVar, -yVar)</p>
  
  <p>where xVar and yVar are character variables (say, with values 'x' and 'y').</p>
  
  <p>Are exclusions simply disallowed with the string versions of functions, or is there a way to do them? </p>
  
  <p>Both of the obvious culprits <code>-yVar</code> and <code>paste0('-', yVar)</code> seem to produce errors.</p>