docs: 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)