This work has been partially supported by NSF grants CCR 92-11732, HRD 93-53271, and INT 95-15256, by NATO Grant CRG 921318, and a fellowship by Phillips Petroleum.
The research has been possible only thanks to the brilliant guidance of Gopal Gupta and his valuable knowledge. It has also benefited from discussions with M. Hermenegildo, M. Carro, V. Santos Costa, I. Dutra, F. Silva, K. Shen, K. Ali, R. Karlsson, and D.H.D. Warren.