code: def formula(self):
          
          formula = super().formula
          chg_str = ""
          if self.charge > 0:
              chg_str = " +" + formula_double_format(self.charge, False)
          elif self._charge < 0:
              chg_str = " " + formula_double_format(self.charge, False)
          return formula + chg_str
docs: Returns a formula string, with elements sorted by electronegativity,
          e.g., Li4 Fe4 P4 O16.