ACTION FOR FAILURE BY A MEMBER STATE TO FULFIL ITS OBLIGATIONS - SUBJECT-MATTER OF THE DISPUTE - DETERMINATION BY A REASONED OPINION - SUBSEQUENT WIDENING OF THE SCOPE OF THE DISPUTE - NOT PERMISSIBLE - TERMINATION OF THE INFRINGEMENT WITHIN THE PERIOD PRESCRIBED IN THE REASONED OPINION - CHALLENGE - ONUS OF PROOF ON THE COMMISSION ( EEC TREATY , ART . 169 )