A workflow started on 1 document stays in 'running' state when the document gets deleted

How to reproduce:

• create a 'File' document
• start a 'Serial document review' workflow on it
• delete the 'File' document
• the workflow stays in 'running' state
• permanently delete the 'File' document
• the workflow stays in 'running' state

The point is that some workflow are "dependent on a Document" and some others are not.

The best solution is probably to let the Workflow itself decide what is it's relationship with the doc.
This means we should simply provide a primitive to add a relation (Core relation or Facet ?) between Doc and WF + a Listener that delete all WF that are related to a deleted doc if they have a "dependent on doc" flag.