Strategy bug found by hypothesis

https://travis-ci.org/nilp0inter/experta/jobs/616530283

```python
state = StrategyStateMachine()
v1 = state.declare(fs={0}, r=0)
v2 = state.declare(fs={0}, r=1)
v3 = state.declare(fs={1}, r=0)
v4 = state.declare(fs={0}, r=2)
state.retract(act=v4)
state.teardown()
```