J. Shapiro. (2007)cite arxiv:0711.3448Comment: Invited Contribution to "The Birth of String Theory" Commemorative Volume. 12 pages, 2 figures. Minor content revision.
R. Bryant. (1999)cite arxiv:math/9910059Comment: 24 pages, plain tex with amssym.tex and amssym.def. To appear in Asterisque. This is the text of a report to the Seminaire Bourbaki in June 1999. Amended to include the new exotic symplectic example of Spin(6,H) in GL(32,R).