Returns a DummySpecie object that can be safely used
        with (i.e. not present in) a given composition

def safe_from_composition(cls, comp: "Composition",
                              oxidation_state: float = 0):
        """
        Returns a DummySpecie object that can be safely used
        with (i.e. not present in) a given composition
        """
        # We don't want to add a DummySpecie with the same
        # symbol as anything in the composition, even if the
        # oxidation state is different
        els = comp.element_composition.elements
        for c in 'abcdfghijklmnopqrstuvwxyz':
            if DummySpecie('X' + c) not in els:
                return DummySpecie('X' + c, oxidation_state)
        raise ValueError("All attempted DummySpecies already "
                         "present in {}".format(comp))