code: def _locate(self, idx):
          
          start = idx * self._width
          end = (idx + 1) * self._width
          sbyte, sbit = divmod(start, 8)
          ebyte = BinInt(end).ceildiv(8)
          return sbyte, sbit, ebyte
docs: Locates an element in the internal data representation.  Returns
          starting byte index, starting bit index in the starting byte, and
          one past the final byte index.