++++ Approximation of laws - Procedures for the award of public works contracts - Directives 71/304/EEC and 71/305/EEC - Registration of contractors - Application by a holding company not itself carrying out the works but availing itself, for the purpose of proving its standing and competence, of references relating to its subsidiaries - Whether permissible - Conditions - Assessment by the national court (Council Directives 71/304 and 71/305)