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: spm_maff.m
    path of file: ./repos/fieldtrip/external/spm8
    
    the code of the file until where you have to start completion: function [M,h] = spm_maff(varargin)
% Affine registration to MNI space using mutual information
% FORMAT M = spm_maff(P,samp,x,b0,MF,M,regtyp,ff)
% P       - filename or structure handle of image
% x       - cell array of {x1,x2,x3}, where x1 and x2 are
%           co-ordinates (from ndgrid), and x3 is a list of
%           slice numbers to use
% b0      - a cell array of belonging probabi
    
function [M,h0] = affreg(buf,MG,x,b0,MF,M,regtyp,ff)
% Do the work

x1 = x{1};
x2 = x{2};
x3 = x{3};
[mu,isig] = priors(regtyp);
isig      =  isig*ff;
Alpha0    =  isig;
Beta0     = -isig*mu;

sol  = M2P(M);
sol1 = sol;
ll   = -Inf;
nsmp = sum(cat(1,buf.nm));
pr   = struct('b',[],'db1',[],'db2',[],'db3',[]);
spm_chi2_plot('Init','Registering','Log-likelihood','Iteration');

for iter=1:200
    penalty = (sol1-mu)'*isig*(sol1-mu);
    T       = MF\P2M(sol1)*MG;
    R       = derivs(MF,sol1,MG);
    y1a     = T(1,1)*x1 + T(1,2)*x2 + T(1,4);
    y2a     = T(2,1)*x1 + T(2,2)*x2 + T(2,4);
    y3a     = T(3,1)*x1 + T(3,2)*x2 + T(3,4);
    h0      = zeros(256,length(b0)-1)+eps;
    for i=1:length(x3),
        if ~buf(i).nm, continue; end