Symbolic diagnostic only

diagnostic only{ For a formal conformal metric $q=(1+\kappa \psi )\eta$ in four dimensions,

Thus its quadratic massless scalar density coefficient vanishes exactly. This independently checks $a_2(\eta \psi )=0$ and the absence of a second local scalar vertex in (eq:sourcedsecond). It is a finite algebraic diagnostic, not a proof of gauge covariance, not a numerical simulation, and not an executed computer-algebra calculation.}