function out=zs(im)

luteven=makelut('zseven',3);
lutodd=makelut('zsodd',3);
done=0;
N=2;
last=im;
previous=applylut(last ,lutodd);
current=applylut(previous,luteven);
while done==0,
    if all(current(:)==last(:)),
        done=1;
    end
    N=N+1;
    last=previous;
    previous=current;
    if mod(N,2)==0,
        current=applylut(current,luteven);
    else
        current=applylut(current,lutodd);
    end;
end;
out=current;