title: "set subtraction"
body: <p>I have three Excel files (actually, csv files, so if you have a solution without Excel that's also great).
  Each file is a list of email addresses.</p>
  
  <p>I want to extract all the emails from file1 that don't appear in files 2 and 3.
  meaning: (file1-file2)-file3 </p>
  
  <p>The only thing I could think of is, the very icky solution of: joining all the files, counting how many times each email appears (<a href="http://office.microsoft.com/en-001/excel-help/count-how-often-a-value-occurs-HP001127779.aspx" rel="nofollow">http://office.microsoft.com/en-001/excel-help/count-how-often-a-value-occurs-HP001127779.aspx</a>), 
  and then filter out all the emails with more then 1 in the count (or that are not from file1).</p>
  
  <p>Has anyone a more elegant solution?</p>