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: plus.m
    path of file: ./repos/chebfun/@chebfun3t
    
    the code of the file until where you have to start completion: function f = plus(f, g)
%PLUS   Addition of two CHEBFUN3T obejects.

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

if ( isempty(f) || isempty(g) ) % CHEBFUN3T + [] = []
    f = [];
    
elseif ( isa(g, 'double') ) % CHEBFUN3T + double
    f.coeffs(1,1,1) = f.coeffs(1,1,1) + g;

    
function f = plus(f, g)
%PLUS   Addition of two CHEBFUN3T obejects.

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

if ( isempty(f) || isempty(g) ) % CHEBFUN3T + [] = []
    f = [];
    
elseif ( isa(g, 'double') ) % CHEBFUN3T + double
    f.coeffs(1,1,1) = f.coeffs(1,1,1) + g;
    f.vscale = abs(f.vscale + g);
    
elseif ( isa(f, 'double') ) % double + CHEBFUN3T
    % Switch argument order and call PLUS again:
    f = plus(g, f);
    
elseif ( isa(f, 'chebfun3t') && isa(g, 'chebfun3t') )  % CHEBFUN3T + CHEBFUN3T
    if (f.domain == g.domain)
        % Make both CHEBFUN3T objects have the same length:
        fCoeffs = f.coeffs;
        gCoeffs = g.coeffs;
        [nxF, nyF, nzF] = size(fCoeffs);
        [nxG, nyG, nzG] = size(gCoeffs);
        fCoeffsNew = zeros(max(nxF,nxG), max(nyF,nyG), max(nzF,nzG));
        gCoeffsNew = zeros(max(nxF,nxG), max(nyF,nyG), max(nzF,nzG));
        fCoeffsNew(1:nxF, 1:nyF, 1:nzF) = fCoeffs;
        gCoeffsNew(1:nxG, 1:nyG, 1:nzG) = gCoeffs;
        
        % Update coefficients:
        f.coeffs = fCoeffsNew + gCoeffsNew;
        f.vscale = vertscale(f);
    else
        error('CHEBFUN3T:plus: Inputs are not defined on the same domain.');
    end