docs: def check_availability(self, isos):
          '''
          This routine checks if the requested set of isotopes is
          available in the dataset.
  
          Parameters
          ----------
          isos : list
              set of isotopes in format ['Si-28','Si-30'].
  
          Returns
          -------
          list
             [index, delta_b, ratio_b].
             index: where is it.
             delta_b: is it a delta value or not?
             ratio_ib:  True if ratio is inverted, false if not
  
          '''
          # make names
          iso1name =  iso_name_converter(isos[0])
          iso2name =  iso_name_converter(isos[1])
  
          ratio = iso1name + '/' + iso2name
          ratio_inv = iso2name + '/' + iso1name
          delta = 'd(' + iso1name + '/' + iso2name + ')'
          delta_inv = 'd(' + iso2name + '/' + iso1name + ')'
  
          index = -1
          # search for data entry
          try:
              index = self.datadict[ratio]
              delta_b = False
              ratio_b = False
          except KeyError:
              try:
                  index = self.datadict[ratio_inv]
                  delta_b = False
                  ratio_b = True
              except KeyError:
                  try:
                      index = self.datadict[delta]
                      delta_b = True
                      ratio_b = False
                  except KeyError:
                      try:
                          index = self.datadict[delta_inv]
                          delta_b = True
                          ratio_b = True
                      except KeyError:
                          index = -1
                          delta_b = None
                          ratio_b = None
          return index, delta_b, ratio_b