def __init__(self, plane_redshifts, cosmology):
self.plane_redshifts = plane_redshifts<EOL>self.cosmology = cosmology<EOL>
Abstract Ray tracer for lens systems with any number of planes.

        From the galaxies of the tracer's planes, their grid-stack(s) and the cosmology physically derived quantities \
        (e.g. surface density, angular diameter distances, critical surface densities) can be computed.

        Parameters
        ----------
        plane_redshifts : [pl.Plane] or [pl.PlaneStack]
            The list of the tracer's planes in ascending redshift order.
        cosmology : astropy.cosmology
            The cosmology of the ray-tracing calculation.