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_cat_struct.m
    path of file: ./repos/ExploreASL/External/SPMmodified
    
    the code of the file until where you have to start completion: function s = spm_cat_struct(s1, s2, varargin)
% Concatenates structure arrays with possibly different fields
% FORMAT s = spm_cat_struct(s1, s2, ...)
%__________________________________________________________________________
% Copyright (C) 2013-2017 Wellcome Trust Centre for Neuroimaging

% Vladimir Litvak
% $Id: spm_cat_struct.m 7074 2017-05-05 10:51:08Z guillaume $


if ~nargin
    s1 = struct([]);
    s2 = struct([]);

    
function s = spm_cat_struct(s1, s2, varargin)
% Concatenates structure arrays with possibly different fields
% FORMAT s = spm_cat_struct(s1, s2, ...)
%__________________________________________________________________________
% Copyright (C) 2013-2017 Wellcome Trust Centre for Neuroimaging

% Vladimir Litvak
% $Id: spm_cat_struct.m 7074 2017-05-05 10:51:08Z guillaume $


if ~nargin
    s1 = struct([]);
    s2 = struct([]);
elseif nargin == 1
    s2 = struct([]);
elseif nargin > 2
    s2 = spm_cat_struct(s2,varargin{:});
end