This Christmas sequence may have originally been written by St Bernard of Clairvaux, although some think it was written earlier. The theory arises in part because of the friendship of Sts Bernard and Norbert, as this sequence was originally part of the Praemonstratensian (Norbertine) liturgy.
This is the Dominican version, which is sung in the Dominican rite 3 times a year: the third Mass on Christmas, Epiphany, and the Presentation of the Lord.
I understand that the sequence "follows" the tune of the Alleluia--hence the name "sequence." I would very much like to be pointed to scholarship about this, and in particular to learn how/ whether the sequences in current use "follow" the melodies of their past or present Alleluias.