Legend suggests the St. Gregory the Great (d. 604) read the very first a few lines chanted by angels on a particular Easter early morning in Rome although he walked barefoot in a great religious procession and which the saint thereupon added the fourth line: “Ora Professional nobis Deum. Alleluia.” https://vitapronobis64208.laowaiblog.com/30413987/vita-pro-nobis-things-to-know-before-you-buy