CONVENTION ON JURISDICTION AND THE ENFORCEMENT OF JUDGMENTS - SPECIAL JURISDICTION - COURT FOR THE PLACE OF PERFORMANCE OF A CONTRACTUAL OBLIGATION - CLAIMS BASED ON DIFFERENT OBLIGATIONS RESULTING FROM A CONTRACT OF EMPLOYMENT - OBLIGATION TO BE TAKEN INTO ACCOUNT FOR THE PURPOSE OF JURISDICTION - OBLIGATION CHARACTERIZING THE CONTRACT IN QUESTION ( CONVENTION OF 27 SEPTEMBER 1968 , ART . 5 ( 1 ))