} else {
if (VertexFunc != null)
vertexset.put(vid,
VertexFunc.reduce(next.vertex().vdata(), VertexFunc.base()));
else
vertexset.put(vid, next.vertex().vdata());
}
} else {
// Apply reduce on edges, remove self and (or merge) duplicate edges.
// Optionally remove bidirectional edge.
Pair p = new Pair(next.edge().source(), next.edge().target());