code: def deflections_of_galaxies_from_sub_grid(sub_grid, galaxies):
      
      if galaxies:
          return sum(map(lambda galaxy: galaxy.deflections_from_grid(sub_grid), galaxies))
      else:
          return np.full((sub_grid.shape[0], 2), 0.0)
docs: Compute the deflections of a list of galaxies from an input sub-grid, by summing the individual deflections \
      of each galaxy's mass profile.
  
      The deflections are calculated on the sub-grid and binned-up to the original regular grid by taking the mean value \
      of every set of sub-pixels.
  
      If no galaxies are entered into the function, an array of all zeros is returned.
  
      Parameters
      -----------
      sub_grid : RegularGrid
          The grid (regular or sub) of (y,x) arc-second coordinates at the centre of every unmasked pixel which the \
          deflections is calculated on.
      galaxies : [galaxy.Galaxy]
          The galaxies whose mass profiles are used to compute the surface densities.