Journal article (2002) - D den Hertog (author), Etienne de Klerk (author), Etienne de de Klerk (author), E. De Klerk (author), Etienne De Klerk (author), Etienne Klerk (author), E. de Klerk (author), Etienne de De Klerk (author), E. Klerk (author), Etienne de Klerk (author), C. Roos (author), C Roos (author), Kees Roos (author)