@classmethod
def add_prefixes(cls, prefix_dict):
    """Add prefixes that will be applied to prefixed units.

        Prefixes do not apply retroactively, that is, they only apply to
        prefixed units created after the prefixes are added.

        Example:
            >>> UnitsGroup.add_prefixes({'ty': [10**-100, teeny]})
        """
    cls._prefixes.update(prefix_dict)
