title: Check whether values in one data frame column exist in a second data frame
body: <p>I have two data frames (A and B), both with a column 'C'. I want to check if values in column 'C' in data frame A exists in data frame B.</p>
  <pre><code>A = data.frame(C = c(1,2,3,4))
  B = data.frame(C = c(1,3,4,7))
  </code></pre>