code: def changed_bit_pos(a, b):
      
      c = a ^ b
      n = 0
      while c > 0:
          if c & 1 == 1:
              return n
          c >>= 1
          n += 1
      return None
docs: Return the index of the first bit that changed between `a` an `b`.
      Return None if there are no changed bits.