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: custom_prior_reconstruction.m
    path of file: ./repos/OMEGA/source
    
    the code of the file until where you have to start completion: function options = custom_prior_reconstruction(options, t, iter, osa_iter)
%% Main reconstruction file for the custom prior file
% This function is used to compute various reconstructions with the
% selected methods and the custom prior gradient.

    
function options = custom_prior_reconstruction(options, t, iter, osa_iter)
%% Main reconstruction file for the custom prior file
% This function is used to compute various reconstructions with the
% selected methods and the custom prior gradient.
%

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Copyright (C) 2020 Ville-Veikko Wettenhovi, Samuli Summala
%
% This program 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.
%
% This program 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 this program. If not, see <https://www.gnu.org/licenses/>.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

if options.OS_bool && options.MLEM_bool
    error('Custom prior reconstruction is not supported when BOTH an OS-type algorithm and a non-OS algorithm are selected')
end