as in [1] footnote 26 (i.e. 10^{-50} is added to denominators $f_i+h_i$$)
as in [2] i.e. for $f_i+h_i < 10^{-4}$ a Taylor expansion is used to avoid zero denominators
as in [1] footnote 26 (i.e. 10^{-50} is added to denominators $f_i+h_i$$)