code: def _read_n(self, l, ret):
          
          
          file = os.path.join(self.dir, l.chr + ".n.1bit")
          
          if not os.path.exists(file):
              return
          
          data = DNA2Bit._read_1bit_file(file, l)
          
          d = DNA2Bit._read1bit(data, l)
          
          for i in range(0, len(ret)):
              if d[i] == 1:
                  ret[i] = DNA_N_UC
docs: Reads 'N' mask from 1 bit file to convert bases to 'N'. In the
          2 bit file, 'N' or any other invalid base is written as 'A'.
          Therefore the 'N' mask file is required to correctly identify where
          invalid bases are.
          
          Parameters
          ----------
          l : tuple
              location
          ret : list
              List of bases which will be modified in place.