code: def _is_from_chemical_system(chemical_system, struct):
          
          chemsys = list(set([sp.symbol for sp in struct.composition]))
          if len(chemsys) != len(chemical_system):
              return False
          for el in chemsys:
              if not el in chemical_system:
                  return False
          return True
docs: checks if the structure object is from the given chemical system