How do I retrieve IPv6 address bits in an iRule

I want to pull bits of an IPv6 client address and manipulate them to create a SNAT address to hand to an IPv4-only pool? I know I don't need to do it this way, but I'm curious how to do the maniupation of the v6 address. I've been searching on devcentral and cannot find any examples of grabbing parts of a v6 address.

eg. 2001:abcd:1:2:3:4:5:6:7:8 gets mapped to 10.ab.cd.78 [aka 10.171.205.120 ]