Judgment of the Court (Third Chamber) of 7 July 1988 in Joined Cases 154 and 155/87: (references for a preliminary ruling made by the Third Chamber of the Hof van Cassatie of Belgium): Rijksinstituut voor de Sociale Verzekeringen der Zelfstandigen v. Heinrich Wolf and Others (Reference for a preliminary ruling — provision of services — conditions for contributions under the Belgian social security scheme for self employed persons — Articles 7 and 52 of the EEC Treaty)