Like his immediate predecessors, Clement was devoted to France, and he demonstrated his French sympathies by refusing a solemn invitation to return to Rome from the city's people, as well as from the poet Petrarch. To placate the Romans, however, Clement VI issued the bull "Unigenitus Dei filius" on 27 January 1343, reducing the interval between one Great Jubilee and the next from 100 years to 50 years. In the document he elaborated for the first time the power of the pope in the use of indulgences. This document would later be used by Cardinal Cajetan in the examination of Martin Luther and his 95 Theses in his trial at Augsburg in 1518. By then, "Unigenitus" was firmly fixed in Canon Law, having been added in the collection called "Extravagantes".