Convenience constructor to make a ConversionElectrode from a
        composition and a phase diagram.

        Args:
            comp:
                Starting composition for ConversionElectrode, e.g.,
                Composition("FeF3")
            pd:
                A PhaseDiagram of the relevant system (e.g., Li-Fe-F)
            working_ion_symbol:
                Element symbol of working ion. Defaults to Li.

def from_composition_and_pd(comp, pd, working_ion_symbol="Li"):
        """
        Convenience constructor to make a ConversionElectrode from a
        composition and a phase diagram.

        Args:
            comp:
                Starting composition for ConversionElectrode, e.g.,
                Composition("FeF3")
            pd:
                A PhaseDiagram of the relevant system (e.g., Li-Fe-F)
            working_ion_symbol:
                Element symbol of working ion. Defaults to Li.
        """
        working_ion = Element(working_ion_symbol)
        entry = None
        working_ion_entry = None
        for e in pd.stable_entries:
            if e.composition.reduced_formula == comp.reduced_formula:
                entry = e
            elif e.is_element and \
                    e.composition.reduced_formula == working_ion_symbol:
                working_ion_entry = e

        if not entry:
            raise ValueError("Not stable compound found at composition {}."
                             .format(comp))

        profile = pd.get_element_profile(working_ion, comp)
        # Need to reverse because voltage goes form most charged to most
        # discharged.
        profile.reverse()
        if len(profile) < 2:
            return None
        working_ion_entry = working_ion_entry
        working_ion = working_ion_entry.composition.elements[0].symbol
        normalization_els = {}
        for el, amt in comp.items():
            if el != Element(working_ion):
                normalization_els[el] = amt
        vpairs = [ConversionVoltagePair.from_steps(profile[i], profile[i + 1],
                                                   normalization_els)
                  for i in range(len(profile) - 1)]
        return ConversionElectrode(vpairs, working_ion_entry, comp)