Clarify Predicate functions expressiveness and flexibility

Hi,

I'm trying to understand the spec in its current form. Could you clarify how expressive predicate functions you intend predicate functions to be? Specifically, can they be defined as arbitrary functions using some simple scripting language? Or is it limited to a few keywords? In either case, can you elaborate on the goals and constrains that informed your current thinking?

Best,
J

Review the current specification's treatment of predicate functions and the question about arbitrary scripting versus a limited keyword set. Clarify the intended expressiveness, goals, and constraints in the specification once the maintainers have decided the direction.

Written by the indexing model from the issue text.