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: writeSPMmat.m
    path of file: ./repos/fieldtrip/external/afni
    
    the code of the file until where you have to start completion: function [err] = writeSPMmat (matin, matout)
%
%   [err] = writeSPMmat (matin, [matout]);
%
%Purpose:
%   Write an ascii version of SPM's .mat files
%   containing a 4x4 transformation matrix.
%   The output of this function can be used
%   with 3dWarp to create AFNI-formatted data from
%   SPM datasets.
%
%Input Parameters:
%   matin : Name of .mat file containing transform
%   matout: (optional) Name of output file.
%           If matout is not specified, the _ASCII
%           extension is added to matin
%
%Output Parameters:
%   err : 0 No Problem
%       : 1 Problems
%
%
%More Info :
%   3dWarp -help
%   https://afni.nimh.nih.gov/ssc/ziad/SUMA/SUMA_doc.htm
%   (search for SPM)
%
%     Author : Ziad Saad
%     Date : Thu Aug 7 11:31:54 EDT 2003
%     SSCC/NIMH/ National Institutes of Health, Bethesda Maryland

    
function [err] = writeSPMmat (matin, matout)
%
%   [err] = writeSPMmat (matin, [matout]);
%
%Purpose:
%   Write an ascii version of SPM's .mat files
%   containing a 4x4 transformation matrix.
%   The output of this function can be used
%   with 3dWarp to create AFNI-formatted data from
%   SPM datasets.
%
%Input Parameters:
%   matin : Name of .mat file containing transform
%   matout: (optional) Name of output file.
%           If matout is not specified, the _ASCII
%           extension is added to matin
%
%Output Parameters:
%   err : 0 No Problem
%       : 1 Problems
%
%
%More Info :
%   3dWarp -help
%   https://afni.nimh.nih.gov/ssc/ziad/SUMA/SUMA_doc.htm
%   (search for SPM)
%
%     Author : Ziad Saad
%     Date : Thu Aug 7 11:31:54 EDT 2003
%     SSCC/NIMH/ National Institutes of Health, Bethesda Maryland


%Define the function name for easy referencing
FuncName = 'writeSPMmat';

%Debug Flag
DBG = 1;

%initailize return variables
err = 1;

%does the input file exist ?
matin = deblank(matin);
matin = deblank(fliplr(matin));
matin = fliplr(matin);

if (exist(matin) ~= 2),
   fprintf(2,'Error %s:\nFile %s not found.\n', FuncName, matin);
   return;
end