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: dki_pa_4d_data2fit.m
    path of file: ./repos/md-dmri/methods/dki_pa
    
    the code of the file until where you have to start completion: function mfs_fn = dki_pa_4d_data2fit(s, mfs_fn, opt, ind)
% function mfs_fn = dki_pa_4d_data2fit(s, mfs_fn, opt, ind)
% 

    
function mfs_fn = dki_pa_4d_data2fit(s, mfs_fn, opt, ind)
% function mfs_fn = dki_pa_4d_data2fit(s, mfs_fn, opt, ind)
% 
% s         - structure with two fields pointing to data (s.nii_fn and s.xps)
% mfs_fn    - model fit structure filename
% opt       - options (optional)
% ind       - singal indices used to fit the model (optional)

if (nargin < 3), opt = []; end