IPv4-translatable addresses: IPv6 addresses to be assigned to IPv6
nodes for use with stateless translation. They have an explicit
mapping relationship to IPv4 addresses. A stateless translator
uses the corresponding IPv4 addresses to represent the IPv6
addresses. A stateful translator does not use this kind of
addresses, since IPv6 hosts are represented by the IPv4 address
pool in the translator via dynamic state.
I can't help you with how to write the eval until you let me know how you'd like the lossy conversion from IPv6 to IPv4 to look like - keep in mind, IPv6 addresses are 128bit while IPv4 ones are only 32bit.