Most full-length works gestate for many years, although maniacally prolific composers like Rossini and Donizetti churned them out at a rate of three or four a year. [...] imagine trying to knock out even a single operatic scene from scratch in the space of just two days. On Friday, April 28, a large collection of composers, librettists, stage directors and singers (including soprano Tanya Roberts) will gather to get their assignments.