title: Find Missing Letter in List (Lowercase or Uppercase, Exclusively)
body: <p>Input is a list, consistently lower or uppercase. Within the sequence, when sorted correctly, one letter is missing. The function should return the missing letter as string output. See code below, where you'll notice I'm halfway done having calculated the missing letter just for lowercase lists. </p>
  
  <pre><code>import string 
  def find_missing_letter(chars):
      for letter in string.ascii_lowercase:
          if letter not in chars:
              return letter[0]
  </code></pre>
  
  <p>Test examples: </p>
  
  <pre><code>test.assert_equals(find_missing_letter(['a','b','c','d','f']), 'e')
  test.assert_equals(find_missing_letter(['O','Q','R','S']), 'P')
  </code></pre>
  
  <p>Anyone know how to check regardless of letter case??</p>