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_maskimage.m
    path of file: ./repos/cat12
    
    the code of the file until where you have to start completion: function out = cat_vol_maskimage(job)
% Po = cat_vol_maskimage(job)
%
% job
%   .data    .. original images
%   .mask    .. lesion/brain mask images
%   .prefix  .. filename prefix 'masked_'
% ______________________________________________________________________
%
% Christian Gaser, Robert Dahnke
% Structural Brain Mapping Group (https://neuro-jena.github.io)

    
function out = cat_vol_maskimage(job)
% Po = cat_vol_maskimage(job)
%
% job
%   .data    .. original images
%   .mask    .. lesion/brain mask images
%   .prefix  .. filename prefix 'masked_'
% ______________________________________________________________________
%
% Christian Gaser, Robert Dahnke
% Structural Brain Mapping Group (https://neuro-jena.github.io)
% Departments of Neurology and Psychiatry
% Jena University Hospital
% ______________________________________________________________________
% $Id$


  def.returnOnlyFilename = 0; 
  def.prefix = 'masked_';
  def.mask   = {''};
  def.bmask  = {''};
  def.recalc = 0; 
  job = cat_io_checkinopt(job,def);

  job.data  = cellstr(job.data);
  job.mask  = cellstr(job.mask);
  job.bmask = cellstr(job.bmask);
  
  % convert to really empty strings
  if numel(job.mask)==1  && isempty(job.mask{1}),  job.mask  = {}; end