. 0. Freund and Y. Nambu, "Scalar field coupled to the trace of the energy-momentum tensor" Phys. Rev. 174 (1968) 1741-1743. Also see Appendix II in R. H. Kraichnan, "Special- Relativistic Derivation of Generally Covariant Gravitation Theory" Phys. Rev. 98 (1955) 1118-1122. The history of this idea is traced in