Predicate signature InputSig::isEvaluatingInOverlay

Holds if the evaluator is currently evaluating with an overlay. The implementation of this predicate needs to be overlay[local]. For a language with no overlay support, none() is a valid implementation.

When called from a local predicate, this predicate holds if we are in the overlay-only local evaluation. When called from a global predicate, this predicate holds if we are evaluating globally with overlay and base both visible.