Bibliographical noteFunding Information:
One of the authors (A. B.) was supported by C.N.R.S. (France) with a N.A.T.O. Research Grant, and the other (J.A.) by an N.I.H. Training Grant, GM-572. This work is part of a research project supported by U.S. Public Health Service Grant NS-2567. Computer facilities were made available by U.S. Air Force Grant AFSC-1221.