docs: def get_residue_id_to_type_map(self):
          '''Returns a dictionary mapping 6-character residue IDs (Chain, residue number, insertion code e.g. "A 123B") to the
             corresponding one-letter amino acid.
  
             Caveat: This function ignores occupancy - this function should be called once occupancy has been dealt with appropriately.'''
  
          resid2type = {}
          atomlines = self.parsed_lines['ATOM  ']
          for line in atomlines:
              resname = line[17:20]
              if resname in allowed_PDB_residues_types and line[13:16] == 'CA ':
                  resid2type[line[21:27]] = residue_type_3to1_map.get(resname) or protonated_residue_type_3to1_map.get(resname)
          return resid2type