2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/127731This paper has been withdrawn by the author, since it is now a part of "Classical aspects of lightlike dimensional reduction" Class.Quant.Grav.23:7085-7110, 2006, gr-qc/0610011This paper has been withdrawn by the author since it is now a part of "Classical aspects of lightlike dimensional reduction" Class.Quant.Grav.23:7085-7110, 2006, gr-qc/0610011General Relativity and Quantum Cosmologytext