old_contents: def find_missing_number(nums):
      """Returns the missing number from a sorted sequence of integers
      in O(n) time and space.
      
      >>> find_missing(i for i in range(-2000, 10000) if i != 1234)
      1234
      """
  
      # Ensure indexable
      nums = list(nums)
  
      # Force odd length
      if len(nums) & 1 == 0:
          nums.append(nums[-1] + 1)
  
      missing = 0
      for i, num in enumerate(nums):
          if num != 0:
              missing ^= num
          missing ^= i + 1
      return missing
  
new_contents: def find_missing_number(nums):
      """Returns the missing number from a sorted sequence of integers
      in O(n) time and space.
  
      >>> find_missing(i for i in range(-2000, 10000) if i != 1234)
      1234
      """
  
      # Ensure indexable
      nums = list(nums)
  
      # Force odd length
      if len(nums) & 1 == 0:
          nums.append(nums[-1] + 1)
  
      missing = 0
      for i, num in enumerate(nums):
          if num != 0:
              missing ^= num
          missing ^= i + 1
  
      return missing
  
subject: fix whitespace