code: def all_combinations_for_elements(self, element_symbols):
          
          d = OrderedDict()
          for symbol in element_symbols:
              d[symbol] = self.select_symbols(symbol, ret_list=True)
  
          from itertools import product
          return list(product(*d.values()))
docs: Return a list with all the the possible combination of pseudos
          for the given list of element_symbols.
          Each item is a list of pseudopotential objects.
  
          Example::
  
              table.all_combinations_for_elements(["Li", "F"])