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: diff.m
    path of file: ./repos/chebfun/@unbndfun
    
    the code of the file until where you have to start completion: function f = diff(f, k, dim)
%DIFF	Derivative of a UNBNDFUN.
%   DIFF(F) is the derivative of F and DIFF(F, K) is the K-th derivative.
%
%   DIFF(F, K, DIM), where DIM is one of 1 or 2, takes the Kth difference along

    
function f = diff(f, k, dim)
%DIFF	Derivative of a UNBNDFUN.
%   DIFF(F) is the derivative of F and DIFF(F, K) is the K-th derivative.
%
%   DIFF(F, K, DIM), where DIM is one of 1 or 2, takes the Kth difference along
%   dimension DIM. For DIM = 1, this is the same as above. For DIM = 2, this is
%   a finite difference along the columns of an array-valued UNBNDFUN. If F has
%   L columns, an UNBNDFUN with an empty ONEFUN property will be returned for K
%   >= L.
%
% See also SUM, CUMSUM.

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

% First derivative by default:
if ( nargin < 2 || isempty(k) )
    k = 1;
end