problem with reading source code-pull out nondeterministic expresssions

[汪洋]

Hi fellas,
I am new to spark and I have a newbie question. I am currently reading the 
source code in spark sql catalyst analyzer. I not quite understand the partial 
function in PullOutNondeterministric. What does it mean by "pull out”? Why do 
we have to do the "pulling out”?
I would really appreciate it if somebody explain it to me. 
Thanks.