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: diffz.m
    path of file: ./repos/chebfun/@chebfun3
    
    the code of the file until where you have to start completion: function g = diffz(f, n)
%DIFFZ   Differentiate a CHEBFUN3 with respect to its third argument.

    
function g = diffz(f, n)
%DIFFZ   Differentiate a CHEBFUN3 with respect to its third argument.
%   G = DIFFZ(F) returns a CHEBFUN3 representing the derivative of F in its 
%   second argument. This is the same as DIFF(F, 1, 3).
%
%   G = DIFFZ(F, N) returns a CHEBFUN3 representing the Nth derivative of F
%   in its second argument. This is the same as DIFF(F, N, 3).
% 
% See also CHEBFUN3/DIFFX, CHEBFUN3/DIFFY, and CHEBFUN3/DIFF.

% Copyright 2017 by The University of Oxford and The Chebfun Developers.
% See http://www.chebfun.org/ for Chebfun information.

% Default to first derivative: 
if ( nargin == 1 ) 
    n = 1; 
end