ZFITTER: a semi-analytical program for fermion pair production in e+e- annihilation, from version 6.21 to version 6.42. - A. B. Arbuzov; Malgorzata Awramik; Michal Czakon; Ayres Freitas; M. W. Grünewald; K. Mönig; Sabine Riemann; Tord Riemann (Comput. Phys. Commun., 2006)