You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: eeg_leadfield4_prepare.m
    path of file: ./repos/fieldtrip/forward/private
    
    the code of the file until where you have to start completion: function [lut_t, cuf_t] = eeg_leadfield4_prepare(vol, order)

% EEG_LEADFIELD4_PREPARE computes constant factors for series expansion
% for the 4 concentric sphere electric leadfield computation. Calling
% this function speeds up subsequent computations, as the constant
% factors "t" do not have to be computed each time in eeg_leadfield4.
%
% Use as

    
function [lut_t, cuf_t] = eeg_leadfield4_prepare(vol, order)

% EEG_LEADFIELD4_PREPARE computes constant factors for series expansion
% for the 4 concentric sphere electric leadfield computation. Calling
% this function speeds up subsequent computations, as the constant
% factors "t" do not have to be computed each time in eeg_leadfield4.
%
% Use as
%   vol.t = eeg_leadfield4_prepare(vol, order);
% where
%   vol.r      radius of the 4 spheres 
%   vol.cond   conductivity of the 4 spheres
% and N is the number of terms for the series (default 60). 
%
% The center of the spheres should be at the origin.
%
% This implementation is adapted from
%   Lutkenhoner, Habilschrift 1992.
% which again is taken from
%   B. N. Cuffin and D. Cohen. Comparion of the Magnetoencephalogram and the Electroencephalogram. Electroencephalogr Clin Neurophysiol, 47:131-146, 1979.
%
% See also EEG_LEADFIELD4

% Copyright (C) 2002, Robert Oostenveld
%
% This file is part of FieldTrip, see http://www.fieldtriptoolbox.org
% for the documentation and details.
%
%    FieldTrip is free software: you can redistribute it and/or modify
%    it under the terms of the GNU General Public License as published by
%    the Free Software Foundation, either version 3 of the License, or
%    (at your option) any later version.
%
%    FieldTrip is distributed in the hope that it will be useful,
%    but WITHOUT ANY WARRANTY; without even the implied warranty of
%    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
%    GNU General Public License for more details.
%
%    You should have received a copy of the GNU General Public License
%    along with FieldTrip. If not, see <http://www.gnu.org/licenses/>.
%
% $Id$

% sort the spheres from the smallest to the largest
[vol.r, indx] = sort(vol.r);
vol.cond      = vol.cond(indx);

r1 = vol.r(1); c1 = vol.cond(1);
r2 = vol.r(2); c2 = vol.cond(2);
r3 = vol.r(3); c3 = vol.cond(3);
r4 = vol.r(4); c4 = vol.cond(4);

if nargin==1 || isempty(order)
  order = 60;
end