title: Accepting all alphabetical input as valid regardless of case
body: <p>How can I alter this code so that regardless of the case of the letters in the argument it will return True? This program just checks to see if the argument exists in any of the lists, so for example if I put <code>this('NORTH')</code>I will get the same result as <code>this('north')</code> or <code>this('nOrTh')</code> Can I use <code>isupper</code> and <code>islower</code>? Thanks in advance. </p>
  
  <pre><code>def scan(words):
      directions = ['north', 'south', 'east', 'west', 'down', 'up', 'left', 'right',  'back']
      verbs = ['go', 'stop', 'kill', 'eat']
      stop_words = ['the', 'in', 'of', 'from', 'at', 'it']
      nouns = ['door', 'bear', 'princess', 'cabinet']
  
  
  lex = words.split()
  list1 = []
  
  for i in lex:
      if i in directions:
          list1.append(('direction', i))
      elif i in verbs:
          list1.append(('verb', i))
      elif i in stop_words:
          list1.append(('stop', i))
      elif i in nouns:
          list1.append(('noun', i))
      elif i.isdigit():
          list1.append(('number', convert_number(i)))
      else:
          list1.append(('error', i))
  return list1 
  
  def convert_number(s):
      try:
          return int(s)
  
      except ValueError:
          return None
  
  this = scan('NORTH')
  print this
  </code></pre>