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_filter.m
    path of file: ./repos/malini/Codes/deconvolution - model/spm8
    
    the code of the file until where you have to start completion: function D = spm_eeg_filter(S)
% Filter M/EEG data
% FORMAT D = spm_eeg_filter(S)
%
% S           - input structure (optional)
% (optional) fields of S:
%   S.D       - MEEG object or filename of M/EEG mat-file
%   S.filter  - struct with the following fields:
%      type       - optional filter type, can be
%                    'but' Butterworth IIR filter (default)
%                    'fir' FIR filter using Matlab fir1 function
%      order      - filter order (default - 5 for Butterworth)
%      band       - filterband [low|high|bandpass|stop]
%      PHz        - cutoff frequency [Hz]
%      dir        - optional filter direction, can be
%                   'onepass'         forward filter only
%                   'onepass-reverse' reverse filter only, i.e. backward in time
%                   'twopass'         zero-phase forward and reverse filter

    
function D = spm_eeg_filter(S)
% Filter M/EEG data
% FORMAT D = spm_eeg_filter(S)
%
% S           - input structure (optional)
% (optional) fields of S:
%   S.D       - MEEG object or filename of M/EEG mat-file
%   S.filter  - struct with the following fields:
%      type       - optional filter type, can be
%                    'but' Butterworth IIR filter (default)
%                    'fir' FIR filter using Matlab fir1 function
%      order      - filter order (default - 5 for Butterworth)
%      band       - filterband [low|high|bandpass|stop]
%      PHz        - cutoff frequency [Hz]
%      dir        - optional filter direction, can be
%                   'onepass'         forward filter only
%                   'onepass-reverse' reverse filter only, i.e. backward in time
%                   'twopass'         zero-phase forward and reverse filter
%                
% D           - MEEG object (also written to disk)
%__________________________________________________________________________
% Copyright (C) 2008 Wellcome Trust Centre for Neuroimaging

% Stefan Kiebel
% $Id: spm_eeg_filter.m 4127 2010-11-19 18:05:18Z christophe $

SVNrev = '$Rev: 4127 $';

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

if nargin == 0
    S = [];
end