ref: 3f316c5c9265618fe7095cc39c4cb10909cbe468
parent: 8ef27e2fe652a8b29a8b57589863f2f2b45f9425
author: Peter Mikkelsen <peter@pmikkelsen.com>
date: Fri Jul 16 16:09:02 EDT 2021
Implement a bit more of prolog flag predicates set_prolog_flag/2 and current_prolog_flag/2