docs: def _stable_names(self):
          '''
          This private method extracts the element names from stable_el.
          Note that stable_names is a misnomer as stable_el also contains
          unstable element names with a number 999 for the *stable* mass
          numbers. (?!??)
  
          '''
          stable_names=[]
          for i in range(len(self.stable_el)):
              stable_names.append(self.stable_el[i][0])
          self.stable_names=stable_names