Legend claims the St. Gregory the Great (d. 604) listened to the first 3 strains chanted by angels on a particular Easter early morning in Rome although he walked barefoot in an awesome religious procession and which the saint thereupon added the fourth line: “Ora Professional nobis Deum. Alleluia.” Almighty, https://vitapronobis97531.blog2freedom.com/31136483/vita-pro-nobis-secrets