FAQ

Why the Frozen base?

This package is designed to ensure that the graphs it builds are always valid, and the operations on them preserve that validity. These graphs are therefore immutable: any change to a graph must prove that the new graph is actually valid too. Furthermore, the checks are additive: every additional level (e.g. executable/runnable) only adds checks, never removes them. This only holds if no subclass accidentally overrides checks or changes what a check reads. Python is rather permissive here, so to make that enforced rather than convention, no implemented method may be overridden in a subclass. You may extend such a class with additional methods (abstract or not), or add additional rules and checks though.

The Frozen base class implements this pattern; it is additionally used for the nodes, since they carry the same proof obligation.