title: How to combine two data frames by equal elements
body: <p>I have two data frames containing the names of genetic elements. I want another data frame with the elements in common in both data frames. 
  Example:</p>
  
  <pre><code>data.a      data.b
     Column     Column
  1  a          c
  2  b          e
  3  c          l
  4  d          a
  </code></pre>
  
  <p>I want this result:</p>
  
  <pre><code>data.c
     Column
  1   a
  2   c
  </code></pre>
  
  <p>This is just an example. The data frame <code>data.b</code> has more elements than <code>data.a</code>. </p>