Find the next meta-interpreter rule instantiation that will fire.
An unbound variable introduced by
This backward chaining goal is only relevant when writing a meta-interpreter for a context. It binds
to the next preferred instantiation from the conflict set of the currently executing context.
This goal may be satisfied repeatedly each time returning the next instantiation. When no instantiations are left, it fails.
KnowledgeWorks and Prolog User Guide (Unix version) - 6 Dec 2011