'Normalizes equivalencies, ensuring each is a 4-tuple of the form::
(from_unit, to_unit, forward_func, backward_func)
Parameters
equivalencies : list of equivalency pairs, or `None`
Returns
A normalized list, including possible global defaults set by, e.g.,
`set_enabled_equivalencies`, except when `equivalencies`=`None`,
in which case the returned list is always empty.
Raises
ValueError if an equivalency cannot be interpreted'

@staticmethod
def _normalize_equivalencies(equivalencies):

    normalized = _normalize_equivalencies(equivalencies)
    if (equivalencies is not None):
        normalized += get_current_unit_registry().equivalencies
    return normalized
