Takashi Nakayama Computer-assisted knowledge acquisition system for synthesis planning. 495-503 1991 31 Journal of Chemical Information and Computer Sciences 4 http://pubs.acs.org/cgi-bin/archive.cgi/jcisd8/1991/31/i04/f-pdf/f_ci00004a011.pdf db/journals/jcisd/jcisd31.html#Nakayama91