docs: def is_stable(self,species):
          '''
          This routine accepts input formatted like 'He-3' and checks with
          stable_el list if occurs in there.  If it does, the routine
          returns True, otherwise False.
  
          Notes
          -----
          this method is designed to work with an se instance from
          nugridse.py. In order to make it work with ppn.py some
          additional work is required.
  
          FH, April 20, 2013.
  
          '''
          element_name_of_iso            = species.split('-')[0]
          try:
              a_of_iso               = int(species.split('-')[1])
          except ValueError: # if the species name contains in addition to the
                             # mass number some letters, e.g. for isomere, then
                             # we assume it is unstable. This is not correct but
                             # related to the fact that in nugridse.py we do not
                             # identify species properly by the three numbers A, Z
                             # and isomeric_state. We should do that!!!!!!
              a_of_iso               = 999
          idp_of_element_in_stable_names = self.stable_names.index(element_name_of_iso)
          if a_of_iso in self.stable_el[idp_of_element_in_stable_names][1:]:
              return True
          else:
              return False