"9 How and where can the outcome of this referral be challenged? There is no mechanism to challenge an opinion of the Enlarged Board of Appeal as it constitutes the highest instance of the EPO. According to the Rules of Procedure of the Boards of Appeal, if a board of appeal considers it necessary to deviate from the opinion of the Enlarged Board of Appeal, it shall refer the matter back to the Enlarged Board of Appeal (Art. 21 RPBA)."
Source: http://www.epo.org/topics/issues/computer-implemented-inventions/referral/cii.html


