Args:
            sp1: First species. A dict of {specie/element: amt} as per the
                definition in Site and PeriodicSite.
            sp2: Second species. A dict of {specie/element: amt} as per the
                definition in Site and PeriodicSite.

        Returns:
            True if sets of occupancies (amt) are equal on both sites.

def are_equal(self, sp1, sp2):
        """
        Args:
            sp1: First species. A dict of {specie/element: amt} as per the
                definition in Site and PeriodicSite.
            sp2: Second species. A dict of {specie/element: amt} as per the
                definition in Site and PeriodicSite.

        Returns:
            True if sets of occupancies (amt) are equal on both sites.
        """
        set1 = set(sp1.element_composition.values())
        set2 = set(sp2.element_composition.values())
        if set1 == set2:
            return True
        else:
            return False