Definition: Permitted extension class

A permitted response is a tempered retarded extension agreeing with (eq:Kubo) off the origin, with scaling degree at most eight, and satisfying:

1. real position-space coefficients;

2. parity-even Lorentz covariance;

3. symmetry in each stress pair and under exchange of the index pairs;

4. transversality in both stress slots.

The mass and stress insertion are fixed. This is an extension class for the selected two-stress response, not a classification of all gravitational counterterms.