15 Furthermore, the papal delegates not only exercised the authority of presidents, but their right to this honor of presiding was recognized by all the fathers of the council, as was shown clearly by the letter sent by the synod to St. Leo 'For you', they wrote, 'showed us benevolence in presiding over us in the persons of those who held your place, as the head over the members' (synod of Chalcedon to St. Leo. Ep . xcviii, PL . liv, 951. Mansi vi, 147).
Source: Sempiternus Rex Christus (Vatican.va)