The author would like to acknowledge helpful discussions with J. R. Barker, R. T. Bate, C. D. Cantrell, D. K. Ferry, K. Hess, K. L. Jensen, N. C. Kluksdahl, A. J. Leggett, R. Lodenkamper, J. H. Luscombe, R. K. Mains, F. J. Narcowich, W. Pötz, M. A. Reed, and L. E. Reichl. This work was supported in part by the Office of Naval Research, the Defense Advanced Research Projects Agency, and the U. S. Army Research Office.