code: def _id_remapper(orig, new):
      
      new_chrom_to_index = {}
      for i_n, (chr_n, _) in enumerate(new):
          new_chrom_to_index[chr_n] = i_n
      remap_indexes = {}
      for i_o, (chr_o, _) in enumerate(orig):
          if chr_o in new_chrom_to_index.keys():
              remap_indexes[i_o] = new_chrom_to_index[chr_o]
      remap_indexes[None] = None
      return remap_indexes
docs: Provide a dictionary remapping original read indexes to new indexes.
  
      When re-ordering the header, the individual read identifiers need to be
      updated as well.