title: logicals of shared rows of two data frames
body: <p>How do I get the logicals of shared rows of two data frames?</p>
  
  <pre><code>&gt; a &lt;- data.frame(x = 1:5, y = 7:11)
  &gt; b &lt;- data.frame(x = c(4, 2, 6, 3, 3, 1), y = c(10, 16, 7, 9, 9, 7))
  &gt; a
    x  y
  1 1  7
  2 2  8
  3 3  9
  4 4 10
  5 5 11
  &gt; b
    x  y
  1 4 10
  2 2 16
  3 6  7
  4 3  9
  5 3  9
  6 1  7
  &gt; a.indices &lt;- c(TRUE, FALSE, TRUE, TRUE, FALSE)
  &gt; b.indices &lt;- c(TRUE, FALSE, FALSE, TRUE, TRUE, TRUE)
  </code></pre>
  
  <p>I would like to return a.indices or b.indices.</p>