def __init__(self, redshift, galaxies, positions, compute_deflections=True, cosmology=None):
self.redshift = redshift<EOL>self.galaxies = galaxies<EOL>self.positions = positions<EOL>if compute_deflections:<EOL><INDENT>def calculate_deflections(pos):<EOL><INDENT>return sum(map(lambda galaxy: galaxy.deflections_from_grid(pos), galaxies))<EOL><DEDENT>self.deflections = list(map(lambda pos: calculate_deflections(pos), self.positions))<EOL><DEDENT>self.cosmology = cosmology<EOL>
A plane represents a set of galaxies at a given redshift in a ray-tracer_normal and the positions of image-plane \
        coordinates which mappers close to one another in the source-plane.

        Parameters
        -----------
        galaxies : [Galaxy]
            The list of lens galaxies in this plane.
        positions : [[[]]]
            The (y,x) arc-second coordinates of image-plane pixels which (are expected to) mappers to the same
            location(s) in the final source-plane.
        compute_deflections : bool
            If true, the deflection-angles of this plane's coordinates are calculated use its galaxy's mass-profiles.