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: combine_visitors.m
    path of file: ./repos/CBIG/external_packages/matlab/default_packages/matlab_bgl
    
    the code of the file until where you have to start completion: function cv = combine_visitors(varargin)
% COMBINE_VISITORS Generate a new visitor by combining existing visitors
%
% cv = combine_visitors(v1, v2, ...) generates a new algorithm visitor that
% works by calling the functions in v1 followed by the functions in v2, and
% so on.  
%
% The value returned by the combined visitor function is the bitwise & of
% all the individual return values.  So, if any visitor requests the
% algorithm to halt, then the algorithm will halt.
%
% Note: using a combined visitor is somewhat slower than writing a custom
% combined visitor yourself.
%
% Example:
%    vis1 = struct();
%    vis1.examine_vertex = @(u) fprintf('vis1: examine_vertex(%i)\n', u);
%    vis2 = struct();

    
function cv = combine_visitors(varargin)
% COMBINE_VISITORS Generate a new visitor by combining existing visitors
%
% cv = combine_visitors(v1, v2, ...) generates a new algorithm visitor that
% works by calling the functions in v1 followed by the functions in v2, and
% so on.  
%
% The value returned by the combined visitor function is the bitwise & of
% all the individual return values.  So, if any visitor requests the
% algorithm to halt, then the algorithm will halt.
%
% Note: using a combined visitor is somewhat slower than writing a custom
% combined visitor yourself.
%
% Example:
%    vis1 = struct();
%    vis1.examine_vertex = @(u) fprintf('vis1: examine_vertex(%i)\n', u);
%    vis2 = struct();
%    vis2.examine_vertex = @(u) fprintf('vis2: examine_vertex(%i)\n', u);
%    combined_vis = combine_visitors(vis1, vis2);
%    load graphs/bfs_example.mat
%    breadth_first_search(A,1,combined_vis);

% David Gleich
% Copyright, Stanford University, 2006-2008

% trivial output
if isempty(varargin)
    error('matlab_bgl:invalidParameter', 'combine_visitors requires at least one argument');
end