docs: def unique_id(self):
          """Generates a tuple that uniquely identifies a `Monomer` in an `Assembly`.
  
          Notes
          -----
          The unique_id will uniquely identify each monomer within a polymer.
          If each polymer in an assembly has a distinct id, it will uniquely
          identify each monomer within the assembly.
  
          The hetero-flag is defined as in Biopython as a string that is
          either a single whitespace in the case of a non-hetero atom,
          or 'H_' plus the name of the hetero-residue (e.g. 'H_GLC' in
          the case of a glucose molecule), or 'W' in the case of a water
          molecule.
  
          For more information, see the Biopython documentation or this
          Biopython wiki page:
          http://biopython.org/wiki/The_Biopython_Structural_Bioinformatics_FAQ
  
          Returns
          -------
          unique_id : tuple
              unique_id[0] is the polymer_id unique_id[1] is a triple
              of the hetero-flag, the monomer id (residue number) and the
              insertion code.
          """
          if self.is_hetero:
              if self.mol_code == 'HOH':
                  hetero_flag = 'W'
              else:
                  hetero_flag = 'H_{0}'.format(self.mol_code)
          else:
              hetero_flag = ' '
          return self.ampal_parent.id, (hetero_flag, self.id, self.insertion_code)