A peculiar form of composition which first appears in the liturgy after the ninth century is the _sequence_ or _prosa_. Apparently this originated in the East and grew out of the custom of writing words as mnemonics under the syllables of the word Alleluja.[61] Gradually it became of such importance that it was detached from the Alleluja and made an independent form. The first examples of the sequence which appear in the Latin church are furnished by Notker Balbulus (830-912), who was responsible for developing it to the proportions of an independent form. Indeed he has been called its inventor. An ancient Irish authority, quoted in the Book of Lismore, says, 'Notker, Abbot of St. Gall’s, invented sequences, and Alleluja after them in the form in which they are.’[62] Among the most famous followers of Notker in the composition of sequences may be mentioned Tutilo or Tuathal (d. 915), an Irish monk of St. Gall’s, Wipo, and Adam de Saint-Victor. The council of Trent suppressed all sequences except five, which are still used by the church. These are the _Victimæ pascali laudes_ of Wipo; the _Lauda Sion Salvatorem_ of St. Thomas Aquinas; the _Dies Iræ_ of Thomas de Celano; the _Stabat Mater_ of Jacques de Benedictis, and the _Veni Sancte Spiritus_. Another peculiar form, practically the antithesis of the sequence in its origin, is the trope, which consists of the dilation of the musical or the literary text by the interjection of complementary phrases. This--at least at first--was probably done to avoid monotony. For instance, instead of singing _Kyrie eleison_ nine times in succession, it was sung as follows: '_Kyrie_ cuncti potens genitor Deus, omni creator _eleison_--fons et origo boni pie luxque perennis, _eleison_. _Kyrie_ salvicet pietas tua nos, bone rector, _eleison_,’ etc. (Tutilo, _Cod. S. Gall_, 484.) All parts of the mass, from the Introit to the Communion, have been decorated with tropes. There has been compiled a list of seventy-eight tropes for the _Kyrie_ alone. Like the sequence the trope developed from an accessory function to an independent form which at one time was practised with much assiduity. V