- document
- Douma, F.A. (author), Menke, M.A. (author), Ruygh, E.F.W. (author) report 1994
- document
- Gerrits, H.J. (author), Ruygh, E.F.W. (author) report 1991
- document
- Ruygh, E.F.W. (author) report 1991
- document
- Ruygh, E.F.W. (author), Hopstaken, C.F.A.M. (author) report 1990