  Details of a recent calculation of O(alpha_s^2) corrections to the decay b ->
c,l,nu_l, taking into account the c-quark mass, are described. Construction of
the expansion in the mass ratio m_c/m_b as well as the evaluation of new
four-loop master integrals are presented. The same techniques are applicable to
the muon decay, mu -> e,nu_mu,anti-nu_e. Analytical results are presented, for
the physical cases as well as for a model with purely-vector couplings.
