J. Barrett, M. Rocek, and R. Williams. (1997)cite arxiv:gr-qc/9710056
Comment: 4 pages, amstex. Revision has minor changes and more precise
conclusions.
S. Mercuri. (2010)cite arxiv:1001.1330
Comment: 91 pages, based on two invited lectures given at the 5th
International School on Field Theory and Gravitation, April 20-24, 2009
Cuiaba city, Brazil.
A. Perez. (2004)cite arxiv:gr-qc/0409061
Comment: Lectures presented at the II International Conference of Fundamental
Interactions, Pedra Azul, Brazil, June 2004. Typos corrected.