W. Langdon. Proceedings of the Eleventh Belgium/Netherlands
Conference on Artificial Intelligence (BNAIC'99), page 255--256. Kasteel Vaeshartelt, Maastricht, Holland, (3-4 November 1999)
W. Langdon. SEN-R9907. Centrum voor Wiskunde en Informatica, CWI, P.O. Box 94079, Kruislaan 413, NL-1090 GB
Amsterdam, The Netherlands, (11 April 1999)
W. Langdon. Proceedings of the Genetic and Evolutionary
Computation Conference, 2, page 1092--1097. Orlando, Florida, USA, Morgan Kaufmann, (13-17 July 1999)
E. Crane, and N. McPhee. Genetic Programming Theory and Practice III, volume 9 of Genetic Programming, chapter 15, Springer, Ann Arbor, (12-14 May 2005)