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: sum2.m
    path of file: ./repos/chebfun/@chebfun3
    
    the code of the file until where you have to start completion: function ff = sum2(f, dims)
%SUM2   Definite integration of a CHEBFUN3 in two variables.
%   G = SUM2(F, DIMS) integrates F over two of the variables X, Y or Z 
%   where DIMS is a row vector containing two of the three indices
%   1, 2 or 3. The output is a 1D CHEBFUN in the remaining variable.

    
function ff = sum2(f, dims)
%SUM2   Definite integration of a CHEBFUN3 in two variables.
%   G = SUM2(F, DIMS) integrates F over two of the variables X, Y or Z 
%   where DIMS is a row vector containing two of the three indices
%   1, 2 or 3. The output is a 1D CHEBFUN in the remaining variable.
%
%   G = SUM2(F) is the same as SUM(F, [1, 2]).
%
% See also CHEBFUN3/SUM, CHEBFUN3/SUM3, CHEBFUN3/CUMSUM, CHEBFUN3/CUMSUM2
% and CHEBFUN3/CUMSUM3.

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

% Empty check: 
if ( isempty(f) ) 
    ff = []; 
    return
end