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: spm_eeg_mask.m
    path of file: ./repos/spm
    
    the code of the file until where you have to start completion: function spm_eeg_mask(S)
% Create a mask image for scalp-level contrasts
% FORMAT spm_eeg_mask(S)
%
% S         - input structure (optional)
% (optional) fields of S:
%    image        - file name of an image containing an unsmoothed 
%                   M/EEG data in voxel-space
%    timewin      - start and end of a window in peri-stimulus time [ms]
%    outfile      - output file name
%__________________________________________________________________________

% Vladimir Litvak
% Copyright (C) 2008-2022 Wellcome Centre for Human Neuroimaging


%-Startup
%--------------------------------------------------------------------------
spm('FnBanner', mfilename);
spm('FigName','M/EEG mask generation'); spm('Pointer','Watch');

%-Input parameters

    
function spm_eeg_mask(S)
% Create a mask image for scalp-level contrasts
% FORMAT spm_eeg_mask(S)
%
% S         - input structure (optional)
% (optional) fields of S:
%    image        - file name of an image containing an unsmoothed 
%                   M/EEG data in voxel-space
%    timewin      - start and end of a window in peri-stimulus time [ms]
%    outfile      - output file name
%__________________________________________________________________________

% Vladimir Litvak
% Copyright (C) 2008-2022 Wellcome Centre for Human Neuroimaging


%-Startup
%--------------------------------------------------------------------------
spm('FnBanner', mfilename);
spm('FigName','M/EEG mask generation'); spm('Pointer','Watch');

%-Input parameters
%--------------------------------------------------------------------------
if ~nargin, S = []; end

if ~isfield(S, 'image')
    [S.image, sts] = spm_select(1, 'image', ...
        'Select an unsmoothed M/EEG image (in voxel-space)');
    if ~sts, return; end