docs: def get_DB_references(self):
          ''' "The DBREF record provides cross-reference links between PDB sequences (what appears in SEQRES record) and
                  a corresponding database sequence." - http://www.wwpdb.org/documentation/format33/sect3.html#DBREF
          '''
  
          _database_names = {
              'GB'    :  'GenBank',
              'PDB'   :  'Protein Data Bank',
              'UNP'   :  'UNIPROT',
              'NORINE':  'Norine',
              'TREMBL': 'UNIPROT',
          }
  
          DBref = {}
          for l in self.parsed_lines["DBREF "]: # [l for l in self.lines if l.startswith('DBREF')]
              pdb_id = l[7:11]
              chain_id = l[12]
              seqBegin = int(l[14:18])
              insertBegin = l[18]
              seqEnd = int(l[20:24])
              insertEnd = l[24]
              database = _database_names[l[26:32].strip()]
              dbAccession = l[33:41].strip()
              dbIdCode = l[42:54].strip()
              dbseqBegin = int(l[55:60])
              idbnsBeg = l[60]
              dbseqEnd = int(l[62:67])
              dbinsEnd = l[67]
  
              DBref[pdb_id] = DBref.get(pdb_id, {})
              DBref[pdb_id][database] = DBref[pdb_id].get(database, {})
              if DBref[pdb_id][database].get(chain_id):
                  if not(DBref[pdb_id][database][chain_id]['dbAccession'] == dbAccession and DBref[pdb_id][database][chain_id]['dbIdCode'] == dbIdCode):
                      raise PDBParsingException('This code needs to be generalized. dbIdCode should really be a list to handle chimera cases.')
              else:
                  DBref[pdb_id][database][chain_id] = {'dbAccession'   :   dbAccession, 'dbIdCode'      :   dbIdCode, 'PDBtoDB_mapping' : []}
  
              DBref[pdb_id][database][chain_id]['PDBtoDB_mapping'].append(
                  {'PDBRange'      :   ("%d%s" % (seqBegin,  insertBegin), "%d%s" % (seqEnd,  insertEnd)),
                  'dbRange'       :   ("%d%s" % (dbseqBegin, idbnsBeg), "%d%s" % (dbseqEnd, dbinsEnd)),
                  }
              )
          return DBref