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 although he walked barefoot in a great religious procession and which the saint thereupon added the fourth line: “Ora Professional nobis Deum. Alleluia.” This https://lorenzoashtg.blogkoo.com/top-guidelines-of-vita-pro-nobis-50794328