# example Obstruction Artifact: Avoidable Doeblin (OG4) in a String-Theory Host Structure

## Overview

RU is a certificate engine for universality and obstructions in constrained nonnegative propagation. Host structures supply finite inputs. Outputs are audited certificates or finite obstruction witnesses.

## What is shipped

| Artifact | Role | Path |
|---|---|---|
| Host model | Same fusion templates, but identity loops are tight (alpha=0) | `sources/ru_bridge/fibonacci_constraintgraph_og4_model.json` |
| RU robust certificate (obstruction) | Doeblin exists, but recurrence fails with an explicit OG4 cycle witness | `sources/ru/robust_certificate_fibonacci_constraintgraph_og4.json` |
| Bridge wrapper | Records the reduction and classifies the claim as obstruction | `sources/ru_bridge/bridge_certificate_fibonacci_constraintgraph_og4.json` |

## The obstruction in one line

A Doeblin block exists (A_tau,A_tau), but it can be avoided forever by staying on the identity loop eI00.

This is the finite, checkable OG4 witness.

## Why this pushes to 

It prevents the most common failure mode of universality writeups:

- stating “mixing should happen” without explaining when it does not.

Here, non-universality is certified as a concrete finite witness.

