if ( focus==null || focus.getTupleCount() == 0 ) return;
Graph g = (Graph)m_vis.getGroup(m_group);
Node f = null;
Iterator tuples = focus.tuples();
while (tuples.hasNext() && !g.containsTuple(f=(Node)tuples.next()))
{
f = null;
}
if ( f == null ) return;
g.getSpanningTree(f);