The Book of Hymns is one prime example of a prosulae. Plainchant was being sung day in and day out by the monks, who had already added some slight melody, but was still not easy to memorize. By adding some rhythm and separation in syllables throughout the melody, a new way to not only memorize the verses, but also present them, was born.
In the early Middle Ages, melodic or textual embellishments added to plainchant were referred to as prosulae. In your own words, summarize Notker's description of