Legend suggests the St. Gregory the Great (d. 604) read the very first a few lines chanted by angels on a particular Easter morning in Rome even though he walked barefoot in a great religious procession and which the saint thereupon additional the fourth line: “Ora Professional nobis Deum. Alleluia.” https://vitapronobis32086.blogdanica.com/31059265/the-best-side-of-vita-pro-nobis