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 even though he walked barefoot in a great spiritual procession and which the saint thereupon additional the fourth line: “Ora Professional nobis Deum. Alleluia.” https://vitapronobis08642.amoblog.com/a-review-of-vita-pro-nobis-53091057