'Given a base (representation instance), determine the unit of the
derivative by removing the representation unit from the component units
of this differential.'

def _get_deriv_key(self, base):

    self._check_base(base)
    for name in base.components:
        comp = getattr(base, name)
        d_comp = getattr(self, u'd_{0}'.format(name), None)
        if d_comp:
            d_unit = (comp.unit / d_comp.unit)
            return str(u.Quantity(1.0, d_unit).si.unit)
    else:
        raise RuntimeError(u'Invalid    representation-differential    match!    Not    sure    how    we    got    into    this    state.')
