if(!dg.containsVertex(child)) {
dg.addVertex(child);
}
//add edge, precondition: vertex key - child are already added to the graph
Edge de = def.createEdge(key, child);
dg.addEdge(de);
}
}
/*
* find the id's participating in the cycle, and return the intersection
* with set of id's which actually produce references.