This can occur in real cases when etype is related to his parent by multiple
optional relations where only one could be filled. We could generate 'OR'
expressions but for now this is not handled.

The compound graph may be used to generate RQL expressions that can be used in
permissions. E.g. when permissions depend on permissions on container.

Use a shortest path algorithm to generate RQL expressions from any etype to the
container of the graph using exclusively mandatory relations.
Make expressions more readable by using variables based on etype name.

and write some tests about graph traversal functions.