title: Counting Occurrences of Letter Combinations
body: <p>I'm trying to count the number of occurrences of each letter combination in a text file:</p>
  
  <p><em>'aa', 'ab',...'zy', 'zz'</em>  </p>
  
  <p>I have managed to be able to count the occurrence of single letters easily enough using collections.Counter and am just wondering if there is a similar approach for 2 letter combinations.</p>
  
  <p>Thanks</p>