  We establish optimal L^p bounds for the nontangential maximal function of the
gradient of the solution to a second order elliptic operator in divergence
form, possibly non-symmetric, with bounded measurable coefficients independent
of the vertical variable, on the domain above a Lipschitz graph in the plane,
in terms of the L^p norm at the boundary of the tangential derivative of the
Dirichlet data, or of the Neumann data.
