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.
P. Tuckey. (1993)cite arxiv:gr-qc/9303011
Comment: 5 pages (plus one figure not included, available from author on
request), Plain Tex, no local preprint number (Only change: omitted
"\magnification" command now replaced).