  We propose a method to non-perturbatively calculate the forward-scattering
matrix elements relevant to inclusive semi-leptonic B meson decays.
Corresponding hadronic structure functions at unphysical kinematics are
accessible through lattice QCD calculation of four-point correlation functions.
The unphysical kinematical point may be reached by analytic continuation from
the physical differential decay rate. A numerical test is performed for the B_s
-> X_c l nu mode in the zero-recoil limit. We use lattice ensembles generated
with 2+1 dynamical quark flavors. The valence charm quark mass is tuned to its
physical value, while the bottom quark mass is varied in the range
(1.56-2.44)m_c. From the numerical results we can identify the contributions of
the ground state D_s^(*) meson as well as those of excited states or continuum
states.
