  It has recently been pointed out that a new class of subleading shape
functions involving B-meson matrix elements of non-local four-quark operators
contributes at order Lambda_{QCD}/m_b to B->X_u+l+nu decay distributions in the
endpoint region. The corresponding functions f_u(omega) and f_v(omega) are
estimated using the vacuum-insertion approximation. A numerical analysis of
various B->X_u+l+nu decay spectra suggests that these power corrections are
very small, below present theoretical uncertainties due to other subleading
shape-function contributions.
