code: def set_symbols(self, symbols, functional=None,
                      sym_potcar_map=None):
          
          del self[:]
          if sym_potcar_map:
              for el in symbols:
                  self.append(PotcarSingle(sym_potcar_map[el]))
          else:
              for el in symbols:
                  p = PotcarSingle.from_symbol_and_functional(el, functional)
                  self.append(p)
docs: Initialize the POTCAR from a set of symbols. Currently, the POTCARs can
          be fetched from a location specified in .pmgrc.yaml. Use pmg config
          to add this setting.
  
          Args:
              symbols ([str]): A list of element symbols
              functional (str): The functional to use. If None, the setting
                  PMG_DEFAULT_FUNCTIONAL in .pmgrc.yaml is used, or if this is
                  not set, it will default to PBE.
              sym_potcar_map (dict): A map of symbol:raw POTCAR string. If
                  sym_potcar_map is specified, POTCARs will be generated from
                  the given map data rather than the config file location.