In LTE how to call gtpv2 library to s1ap libray structure,encode and decode?

Question

In srvcc ps to cs request sent mme to msc,14 IE's are there request message but two IE's structure encode and decode are defined S1AP but sv interface ps to cs request message used gtpv2,my doubt was how to call that structure and encode,decode gtpv2 to s1ap.