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: mars_options.m
    path of file: ./repos/bspm/thirdparty/spm12/toolbox/marsbar-0.44
    
    the code of the file until where you have to start completion: function [mars, msgstr] = mars_options(optstr, mars, cfg_fname)
% options utility routines
% FORMAT [mars msgstr] = mars_options(optstr, mars, cfg_fname)
%
% Input [default]
% optstr            - option string: one of
%                     'put','load','save','edit','defaults',
%                     'basedefaults','fill' [load]  
% mars              - marsbar options structure [MARS.OPTIONS]
% cfg_fname         - filename for configuration file [GUI]
% 
% Output
% mars              - possible modified MARS.OPTIONS structure

    
function [mars, msgstr] = mars_options(optstr, mars, cfg_fname)
% options utility routines
% FORMAT [mars msgstr] = mars_options(optstr, mars, cfg_fname)
%
% Input [default]
% optstr            - option string: one of
%                     'put','load','save','edit','defaults',
%                     'basedefaults','fill' [load]  
% mars              - marsbar options structure [MARS.OPTIONS]
% cfg_fname         - filename for configuration file [GUI]
% 
% Output
% mars              - possible modified MARS.OPTIONS structure
% msgstr            - any relevant messages
%
% Matthew Brett 20/10/00,2/6/01
%
% $Id$
  
if nargin < 1
  optstr = 'load';
end