title: Subset by row and column reciprocity
body: <p>I am having trouble subsetting a data.frame based on reciprocity of values in row and column. </p>
  
  <p>Here is a example df to illustrate the problem:</p>
  
  <pre><code>rater &lt;- c(21, 23, 26, 24)
  ratee &lt;- c(24, 21, 23, 21)
  rating.data &lt;- data.frame(rater, ratee)
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>   rater ratee
  1    21    24
  2    23    21
  3    26    23
  4    24    21
  </code></pre>
  
  <p>I would like to subset this df by only keeping the rows that have reciprocal values. </p>
  
  <p>The resulting subset should look like this:</p>
  
  <pre><code>   rater ratee
  1    21    24
  4    24    21
  </code></pre>
  
  <p>Any thoughts would be much appreciated! </p>