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: jsonopt.m
    path of file: ./repos/jsonlab
    
    the code of the file until where you have to start completion: function val = jsonopt(key, default, varargin)
%
% val=jsonopt(key,default,optstruct)
%
% setting options based on a struct. The struct can be produced
% by varargin2struct from a list of 'param','value' pairs
%

    
function val = jsonopt(key, default, varargin)
%
% val=jsonopt(key,default,optstruct)
%
% setting options based on a struct. The struct can be produced
% by varargin2struct from a list of 'param','value' pairs
%
% authors:Qianqian Fang (q.fang <at> neu.edu)
%
% input:
%      key: a string with which one look up a value from a struct
%      default: if the key does not exist, return default
%      optstruct: a struct where each sub-field is a key
%
% output:
%      val: if key exists, val=optstruct.key; otherwise val=default
%
% license:
%     BSD or GPL version 3, see LICENSE_{BSD,GPLv3}.txt files for details
%
% -- this function is part of JSONLab toolbox (http://iso2mesh.sf.net/cgi-bin/index.cgi?jsonlab)
%

val = default;
if (nargin <= 2)
    return
end