Returns an EntrySet containing only the set of entries belonging to
        a particular chemical system (in this definition, it includes all sub
        systems). For example, if the entries are from the
        Li-Fe-P-O system, and chemsys=["Li", "O"], only the Li, O,
        and Li-O entries are returned.

        Args:
            chemsys: Chemical system specified as list of elements. E.g.,
                ["Li", "O"]

        Returns:
            EntrySet

def get_subset_in_chemsys(self, chemsys: List[str]):
        """
        Returns an EntrySet containing only the set of entries belonging to
        a particular chemical system (in this definition, it includes all sub
        systems). For example, if the entries are from the
        Li-Fe-P-O system, and chemsys=["Li", "O"], only the Li, O,
        and Li-O entries are returned.

        Args:
            chemsys: Chemical system specified as list of elements. E.g.,
                ["Li", "O"]

        Returns:
            EntrySet
        """
        chemsys = set(chemsys)
        if not chemsys.issubset(self.chemsys):
            raise ValueError("%s is not a subset of %s" % (chemsys,
                                                           self.chemsys))
        subset = set()
        for e in self.entries:
            elements = [sp.symbol for sp in e.composition.keys()]
            if chemsys.issuperset(elements):
                subset.add(e)
        return EntrySet(subset)