code: def Hill(self):
          r'''Hill formula of a compound. For a description of the Hill system,
          see :obj:`thermo.elements.atoms_to_Hill`.
  
          Examples
          --------
          >>> Chemical('furfuryl alcohol').Hill
          'C5H6O2'
          '''
          if self.__Hill:
              return self.__Hill
          else:
              self.__Hill = atoms_to_Hill(self.atoms)
              return self.__Hilf Hill(self):
          r'''Hill formula of a compound. For a description of the Hill system,
          see :obj:`thermo.elements.atoms_to_Hill`.
  
          Examples
          --------
          >>> Chemical('furfuryl alcohol').Hill
          'C5H6O2'
          '''
          if self.__Hill:
              return self.__Hill
          else:
              self.__Hill = atoms_to_Hill(self.atoms)
              return self.__Hill
docs: r'''Hill formula of a compound. For a description of the Hill system,
          see :obj:`thermo.elements.atoms_to_Hill`.
  
          Examples
          --------
          >>> Chemical('furfuryl alcohol').Hill
          'C5H6O2'