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: cfg_const.m
    path of file: ./repos/ExploreASL/External/SPMmodified/matlabbatch/@cfg_const
    
    the code of the file until where you have to start completion: function item = cfg_const(varargin)

% This is the const configuration item class
%
% Data structure
% ==============
% Description fields
%    * name  - display name of config item
%    * tag   - tag of the menu item
%    * val   - 1x1 cell array
%    * check - (optional) function handle to implement configuration
%              specific subsasgn checks. This is not used here, since a
%              const should not change.
%    * help  - help text
% GUI/job manager fields
%    * expanded
%    * hidden
% All fields are inherited from the generic configuration item class.
%
% Public Methods
% ==============
%    * get_strings - returns name of object
%    * gettag      - returns tag
%    * help        - returns help text
%    * harvest
%    * all_set
%
% * 'const' - Constant value

    
function item = cfg_const(varargin)

% This is the const configuration item class
%
% Data structure
% ==============
% Description fields
%    * name  - display name of config item
%    * tag   - tag of the menu item
%    * val   - 1x1 cell array
%    * check - (optional) function handle to implement configuration
%              specific subsasgn checks. This is not used here, since a
%              const should not change.
%    * help  - help text
% GUI/job manager fields
%    * expanded
%    * hidden
% All fields are inherited from the generic configuration item class.
%
% Public Methods
% ==============
%    * get_strings - returns name of object
%    * gettag      - returns tag
%    * help        - returns help text
%    * harvest
%    * all_set
%
% * 'const' - Constant value
%   - required fields: 'type', 'name', 'tag', 'val'
%   - optional fields: 'help'
%
%   The resulting data structure simply contains the contents of
%   val{1}.
%
% The layout of the configuration tree and the types of configuration items
% have been kept compatible to a configuration system and job manager
% implementation in SPM5 (Statistical Parametric Mapping, Copyright (C)
% 2005 Wellcome Department of Imaging Neuroscience). This code has been
% completely rewritten based on an object oriented model of the
% configuration tree.
%
% This code is part of a batch job configuration system for MATLAB. See 
%      help matlabbatch
% for a general overview.
%_______________________________________________________________________
% Copyright (C) 2007 Freiburg Brain Imaging

% Volkmar Glauche
% $Id: cfg_const.m 5678 2013-10-11 14:58:04Z volkmar $

rev = '$Rev: 5678 $'; %#ok

myclass = mfilename;
% Get local fields and defaults from private/mysubs_fields
[fn, defs] = mysubs_fields;

if nargin == 1
    if isstruct(varargin{1})
        % assume input is a struct to be converted back into a class
        % return with error if this does not work
        if numel(fieldnames(varargin{1})) == numel(fn)+2 && ...
                all(isfield(varargin{1}, [fn(:)', {'cfg_item' 'cfg_leaf'}]))
            gitem = varargin{1}.cfg_item;
            sitem = rmfield(varargin{1},{'cfg_item', 'cfg_leaf'});
            item  = class(sitem, myclass, gitem, cfg_leaf);
            return;
        else
            cfg_message('matlabbatch:constructor:reclassify', ['Don''t know how to convert this ' ...
                            'into class ''%s''.'], myclass);
        end