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: CreateProceduralCheckerboard.m
    path of file: ./repos/opticka/stimuli
    
    the code of the file until where you have to start completion: function [id, rect, shader] = CreateProceduralCheckerboard(windowPtr, width, height, radius)
% [id, rect, shader] = CreateProceduralCheckerboard(windowPtr, width, height, [, radius=inf])
%
% A procedural checkerboard shader
%
% See also: CreateProceduralSineGrating, CreateProceduralSineSquareGrating


    
function [id, rect, shader] = CreateProceduralCheckerboard(windowPtr, width, height, radius)
% [id, rect, shader] = CreateProceduralCheckerboard(windowPtr, width, height, [, radius=inf])
%
% A procedural checkerboard shader
%
% See also: CreateProceduralSineGrating, CreateProceduralSineSquareGrating

% History: 
% 06/06/2014 ima created by Ian Max Andolina <http://github.com/iandol>, licenced under the MIT Licence

global GL;

AssertGLSL;

if nargin < 1 || isempty(windowPtr) 
    error('You must provide a PTB window pointer!');
end