code: def find_one(self):
          
          num = quotient = 0
          while num < self._len:
              chunk = self.data[quotient]
              if chunk & self.one_mask:
                  remainder = 0
                  while remainder < self.width and num < self._len:
                      item = (chunk >> remainder) & 3
                      if item == PC_ONE:
                          return num
                      remainder += 2
                      num += 1
              else:
                  num += (self.width >> 1)
              quotient += 1
          return None
docs: Return the first index of an entry that is either one or DC.
          If no item is found, return None.