  We prove global stability for a system of nonlinear wave equations satisfying
a generalized null condition. The generalized null condition allows for null
forms whose coefficients have bounded $C^k$ norms. We prove both pointwise
decay and improved decay of good derivatives using bilinear energy estimates
and duality arguments. Combining this strategy with the $r^p$ estimates of
Dafermos--Rodnianski then allows us to prove global stability. The proof
requires analyzing the geometry of intersecting null hypersurfaces adapted to
solutions of wave equations.
