# Returns a GraphViz string representing this digraph.
	fun to_dot: String
	do
		var s = "digraph \{\n"
		var id_set = new HashMap[V, Int]
		# Writing the vertices
		for u in vertices_iterator, i in [0 .. vertices.length[ do
			id_set[u] = i
			s += "   \"{i}\" "
			s += "[label=\"{u.to_s.escape_to_dot}\"];\n"
		end
		# Writing the arcs
		for arc in arcs do
			s += "   {id_set[arc[0]]} "
			s += "-> {id_set[arc[1]]};"
		end
		s += "\}"
		return s
	end