old_contents: def missing_number(l):
      non_missing = sum([i for i in range(1,len(l)+2)])
      missing = sum([i for i in l])
      return non_missing - missing
  
  print missing_number([1,2,3,4,6])
  
new_contents: def missing_number(l):
      non_missing = sum(range(1,len(l)+2))
      missing = sum(l)
      return non_missing - missing
  
  print missing_number([1,2,3,4,6])
  
subject: Update missing-number.py (#59)