title: Cannot coerce type 'closure' to vector of type 'double' (polygon)
body: <pre><code>plot( dnorm , col='white')
  polygon( dnorm, col='grey' )
  </code></pre>
  
  <p>returns the above error message, not on <code>plot</code>, but on <code>polygon</code>.</p>
  
  <hr>
  
  <p><code>body(polygon) %&gt;% grep(pattern='numeric')</code> finds only one occurrence on line 4, which doesn't seem to have anything to do with this error. So I'm at a loss as to where to look for the source of the problem.</p>