title: Check if one column value is in another column in pandas
body: <p>I want to compare one column with another column in the same dataframe. Not just between adjacent value but iterating through every value in Col1 and see if it exists in Col2 values.</p>
  
  <pre><code>Col1        Col2    exists
  cat         pig     true
  a           cat     false
  pig         b       true
  mat         axe     false
  </code></pre>
  
  <p>Thanks.</p>