Legend claims the St. Gregory the Great (d. 604) listened to the first 3 lines chanted by angels on a particular Easter early morning in Rome although he walked barefoot in a great religious procession and that the saint thereupon added the fourth line: “Ora Professional nobis Deum. Alleluia.” Almighty, https://vitapronobis37158.webbuzzfeed.com/31671023/not-known-facts-about-vita-pro-nobis