To you, therefore, most merciful Father, we make humble prayer and petition through Jesus Christ, your Son, our Lord: that you accept and bless these gifts, these offerings, these holy and unblemished sacrifices, which we offer you first of all for your holy catholic Church. Be pleased to grant her peace, to guard, unite and govern her throughout the whole world, together with [una cum] your servant N. our Pope and N. our Bishop, and all those who, holding to the truth, hand on the catholic and apostolic faith.Now if you believe that the recent Popes have all been heretics and that the see of Peter is thus vacant, you don't want to be praying in union with the arch-heretic Pope Benedict so you omit the "una cum" clause.
Rorate Caeli has picked up an example of a priest who may not realise who he allied himself with.
French priest, Fr Gérard Bessière, has been so upset by the lifting of the excommunications of the Bishops of the SSPX that he has decided that he will henceforth excommunicate the Pope and not mention him in the Eucharistic Prayer.
I post this as a charitable warning to sedevacantists everywhere. It may be a non-una-cum-Mass but you probably don't want to be there.