title: Pattern finding in a list of character vectors
body: <p>There are vectors with characters of differing length like:</p>
  
  <pre><code>f1 &lt;- c("YTOI", "JKLO", "POLI")
  f2 &lt;- c("KOPL", "KOPL", "KOPL")
  f3 &lt;- c("JNLI", "NJLI", "IUYP")
  f4 &lt;- c("FIPN", "JHPL", "UIPE")
  </code></pre>
  
  <p>They are in a list as <code>fe &lt;- list(f1,f2,f3,f4)</code>. The entries don't all have to be length 3 - could be varying.</p>
  
  <p>How to find those entries (index) where all entries are identical?</p>