In Gas laser device/Mitsubishi, the patentee, whose patent had been upheld by the Opposition Division, sought at the hearing of an appeal by the opponent to introduce the subject matter of one of the subclaims into the main claim as an auxiliary petition for maintenance of the patent. The Appeal Board refused to consider this request and revoked the patent. The Board pointed out that it was well established practice in the EPO that an opponent had only to show that a single claim was invalid for the whole of the set of claims containing it to be unpatentable. The system of filing auxiliary petitions gives the patentee adequate means for preparing "fall-back" positions if its main set of claims is unpatentable. The Board commented "an opponent has no obligation to substantiate grounds of opposition against individual dependent claims ... just in case the patent proprietor may think in the future of filing a further request with the subject matter of such a dependent claim becoming the subject matter of such further request". Since the traditional view of subclaims is that they provide for a fall back position if the main claim is found to be invalid, it seems likely that in the future it will be prudent to file auxiliary petitions in EPO opposition proceedings including the features of a number of different subclaims in order to ensure that this line of retreat is not cut off.