func (o orderSyms) Less(i, j int) bool { return o.less(o.v[i], o.v[j]) }
func (o orderSyms) Swap(i, j int)      { o.v[i], o.v[j] = o.v[j], o.v[i] }

// nodesPerSymbol classifies nodes into a group of symbols.
func nodesPerSymbol(ns graph.Nodes, symbols []*objSymbol) map[*objSymbol]graph.Nodes {
	symNodes := make(map[*objSymbol]graph.Nodes)
	for _, s := range symbols {
		// Gather samples for this symbol.