Commentary Purg XXVII 55-57

The scene we expect, as the protagonist completes his ritual purging on a terrace, is the removal of one (in this case the seventh and last) P from his forehead.  Apparently the poet decided to avoid representing this climactic moment, allowing it either to be intrinsic, or else perhaps allowing us to believe that the fire itself cleansed the protagonist of his predisposition toward lust.  But this is the moment (not before the entrance to the fire, as some believe) at which the ritual act should be performed, just before the upward movement to the next area of the mountain, as it has been on the other terraces (see an associated discussion of the angels' recitations of the Beatitudes in C.Purg.XV.38-39).

      Trucchi (DDP Trucchi.Purg.XXVII.10-15) counts the angels who serve God's ends in Purgatorio and comes up with ten (the 'perfect number,' as he remarks): the Christian Mercury who brings the living dead to the mountain ([Purg II 43]), the angelic warder at the gate ([Purg IX 104]), the seven who are associated with the virtues opposed to the vices repented at the end of the experience of each terrace, and now this one (he omits the two angelic actors in the pageant at [Purg VIII 25-39]).

      This final angel of purgatory (in paradise we see Gabriel circling the Virgin [[Par XXIII 103]] and then the angelic hierarchy in Paradiso XXVIII) acts as a sort of positive Siren, drawing souls away from lust and setting them free.