Rooy, P. de. 2003. ā€œW.J.M. Klaassen, De Progressieve Samenwerking Van PvdA, Dā€™66, PPR En PSP, 1966-1977ā€. BMGN - Low Countries Historical Review 118 (2): 283-84. https://doi.org/10.18352/bmgn-lchr.5877.