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: cat_vol_set_com.m
    path of file: ./repos/cat12
    
    the code of the file until where you have to start completion: function Affine = cat_vol_set_com(V)
% use center-of-mass (COM) to roughly correct for differences in the
% position between image and template
% ______________________________________________________________________
% FORMAT:  Affine = cat_vol_set_com(varargin)
%
% V      - mapped images or filenames 
% Affine - affine transformation to roughly correct origin 
% 
% Only if no input is defined the function is called interactively and the
% estimated transformation is applied to the images. Otherwise, only the 
% Affine paramter is returned.
% ______________________________________________________________________
%
% Christian Gaser, Robert Dahnke
% Structural Brain Mapping Group (https://neuro-jena.github.io)
% Departments of Neurology and Psychiatry
% Jena University Hospital
% ______________________________________________________________________
% $Id$


    
function Affine = cat_vol_set_com(V)
% use center-of-mass (COM) to roughly correct for differences in the
% position between image and template
% ______________________________________________________________________
% FORMAT:  Affine = cat_vol_set_com(varargin)
%
% V      - mapped images or filenames 
% Affine - affine transformation to roughly correct origin 
% 
% Only if no input is defined the function is called interactively and the
% estimated transformation is applied to the images. Otherwise, only the 
% Affine paramter is returned.
% ______________________________________________________________________
%
% Christian Gaser, Robert Dahnke
% Structural Brain Mapping Group (https://neuro-jena.github.io)
% Departments of Neurology and Psychiatry
% Jena University Hospital
% ______________________________________________________________________
% $Id$


if nargin == 1
  if isstruct(V)
    V = V;
  else
    P = char(V);
    V = spm_vol(P);
  end