In principle, there exists a second invariant in the fourth-order derivatives of ~. However, in a system with translational invariance this term is identical with that given in (1). See K. G. Wilson and J. Kogut, Phys. Bep. 12C, 75 (1974); S.-k. Ma, Bev. Mod. Phys. 45, 589 (1973); M. E. Fisher, Bev. Mod. Phys. 46, 597 (1974); and A. Aharony, in "Phase Transitions and Critical Phenomena, " edite...