docs: def add_code_mapping(self, from_pdb_code, to_pdb_code):
          '''Add a code mapping without a given instance.'''
  
          # Consistency check - make sure that we always map the same code e.g. 'LIG' to the same code e.g. 'GTP'
          if from_pdb_code in self.code_map:
              assert(self.code_map[from_pdb_code] == to_pdb_code)
          else:
              self.code_map[from_pdb_code] = to_pdb_code