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: xyz.m
    path of file: ./repos/bspm/thirdparty/spm12/toolbox/marsbar-0.44/@marsy
    
    the code of the file until where you have to start completion: function [XYZ, M]= xyz(o, r_no, xyz_type)
% gets XYZ coordinates for region 
% FORMAT [XYZ M]= xyz(o, r_no, xyz_type)
% 
% Inputs
% o         - marsy object
% r_no      - region number
% xyz_type  - string, one of 'mm','real','vox' 
%             where 'real' is a synonym for 'mm'
%             and 'mm' is the default (if not passed)
%             'mm' results in coordinates in mm
%             'vox' gives coordinates in voxels
% 

    
function [XYZ, M]= xyz(o, r_no, xyz_type)
% gets XYZ coordinates for region 
% FORMAT [XYZ M]= xyz(o, r_no, xyz_type)
% 
% Inputs
% o         - marsy object
% r_no      - region number
% xyz_type  - string, one of 'mm','real','vox' 
%             where 'real' is a synonym for 'mm'
%             and 'mm' is the default (if not passed)
%             'mm' results in coordinates in mm
%             'vox' gives coordinates in voxels
% 
% Outputs
% XYZ       - coordinates in specified reference
% M         - 4x4 transformation mapping voxels to mm
% 
% $Id$

r = n_regions(o);
if nargin < 2
  error('Need region number to get XYZ coordinates')
end