Establish a convention for public/private

[jzelinskie]

Problem Statement

I often want to signal to my fellow developers that a particular permission is "private". This is very common with "synthetic permissions" that exist only to further nest arrows.

This problem will only get exacerbated once there is a module system for composing schemas via library.

Ideally there is an officially sanctioned syntax for representing a private definition.

Solution Brainstorm

Enable identifiers to be able to begin with an underscore