1984 · Nanno H. Mulder, Dirk Th. Sleijfer, et al. · University of Groningen research database (University of Groningen / Centre for Information Technology) · 2
1986 · Nanno H. Mulder, Dirk Th. Sleijfer, et al. · University of Groningen research database (University of Groningen / Centre for Information Technology) · 1