Legend suggests the St. Gregory the Great (d. 604) listened to the very first 3 lines 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.” https://doresnosnervos10975.blogstival.com/53409968/the-best-side-of-vita-pro-nobis