CONVENTION ON JURISDICTION AND THE ENFORCEMENT OF JUDGMENTS - PROROGATION OF JURISDICTION - ENTERING AN APPEARANCE WITHOUT CONTESTING THE JURISDICTION OF THE COURT SEISED OF THE PROCEEDINGS - CLAIM FOR A SET-OFF MADE BY THE DEFENDANT - DISPUTE ON THE SUBSTANCE OF THE CASE ENTERED INTO BY THE APPLICANT - APPLICATION OF ARTICLE 18 - CLAUSE CONFERRING JURISDICTION ON ANOTHER COURT - NO EFFECT ( CONVENTION OF 27 SEPTEMBER 1968 , ARTS 17 AND 18 )