The "nest" instruction was created in the early days of the Guacamole protocol as a means of splitting up large instructions. At the time, this was necessary because images and audio were transmitted within instructions that required the entire block of data to be available at once. The "nest" instruction thus provided a means of streaming data for instructions that otherwise did not support it.
The Guacamole protocol switched over instructions with actual stream semantics around the 0.9.0 release (back in 2014). Since that time, the nest instruction has become unnecessary, and is actually no longer used in practice.
It's time the instruction and related functions/structures were deprecated.