Legend states the St. Gregory The nice (d. 604) listened to the first a few traces chanted by angels on a certain Easter early morning in Rome even though he walked barefoot in an excellent spiritual procession and the saint thereupon added the fourth line: “Ora pro nobis Deum. Alleluia.” https://vita-pro-nobis33197.blogerus.com/53665730/vita-pro-nobis-no-further-a-mystery