function Fy = getprefy(F,y,m,n)
MN = repmat([m],[1 n]);
Fy = [];
mn = m^n;
ly = length(y);
for ind = 1:mn
  z = ind2coord(ind,MN);
  if veq(z(1:ly),y)
    Fy(end+1) = F(ind);
  end
end
Fy = reshape(Fy,size(F(1:length(Fy))));
return