def apply_vcc(self, vcc, distribution_skip=False,
                  **kwargs):
        """
        Applies "velocity contrast curve" to population.

        That is, the constraint that comes from not seeing two sets
        of spectral lines in a high resolution spectrum.

        Only works if population has ``dmag`` and ``RV`` attributes.

        :param vcc:
            Velocity contrast curve; dmag vs. delta-RV.
        :type cc:
            :class:`VelocityContrastCurveConstraint`

        :param distribution_skip:
            This is by default ``True``.  *To be honest, I'm not
            exactly sure why.  Might be important, might not
            (don't remember).*

        :param **kwargs:
            Additional keyword arguments passed to
            :func:`StarPopulation.apply_constraint`.

        """
        rvs = self.RV.value
        dmags = self.dmag(vcc.band)
        self.apply_constraint(VelocityContrastCurveConstraint(rvs,dmags,vcc,
                                                              name='secondary spectrum'),
                              distribution_skip=distribution_skip, **kwargs)