Published in Phys. Rev. D 57 (15 April, 1998) and based
on part of a report given in the session ``Quantum Fields in Curved Space
I'' originally published in the Proc. of the Eighth Marcel Grossmann
Meeting on General Relativity, Jerusalem, June 23-27, 1997, edited by
Tsvi Piran (World Scientific, Singapore, 1998)
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