One of the suggestions mentioned to account for the original lengthening of the _alleluia_ in the _iubilus_ is connected with the trope. The word has long been defined as a textual interpolation.[9] Gastoué, however, contends that it was originally and primarily musical, a vocalization in the existing chant and that it was created in the music school. The ancient form of trope is a _neuma triplex_ added to the response _In medio_ _etc._ for the Feast of St. John the Apostle, or to _Descendit de caelis_ for Christmas. This vocalism is described by Amalarius of Metz and indeed Metz may be its place of origin. Alcuin has been named as the possible originator, a theory strengthened by the fact that Amalarius was one of his pupils.[10] At any rate Amalarius seems to have been the first to call the melody following the _alleluia_, a _sequentia_,[11] from which it is evident that the _iubili_ must have been regarded in some other light prior to his writing. The _sequentia_ in connection with the _alleluia_ may very reasonably have been considered a trope, since vocalisms like these had already appeared elsewhere in rites of worship, and sequences in addition to those which belong to the _alleluia_ of the mass have been found in antiphonaries. To repeat, Gastoué describes a musical interpolation or trope originating in the music schools of the Franks and appearing in various liturgical settings. He likens the _iubilus_ to a trope which Amalarius called a _sequentia_. The original divisions created by the musical phrases in the _iubilus_ now appear in a series, each repeated a certain number of times with introduction and conclusion and thus the completed sequence structure comes into being. The germ of its formal construction, Gastoué finds in certain Gregorian sources. The ancient _alleluia_, _Justus ut palma florebit_, shows such characteristics and reveals the liturgical Latin origin of the sequence, its melody going back to the _versus alleluiaticus_.