[graph-tool] clabel usage for partial constraints

Hi, I have a question regarding the implementing partial constraints during blockmodel minimization. By partial constraints I mean that I have a group of vertices V1 that are pre-labeled into k clusters i.e. vertices from V1 that are in different clusters should not mix. Then there is a second set of vertices V2 that should be able to mix freely with vertices from both V1 and V2. I have tried defining a corresponding vertex property and passing it to clabel argument but have always ended up with partitions in which V1 and V2 don't mix in the final state. Best regards, Anatol