def add_n_vertices(n, *args)
      vertices = []
      n.times do |i|
        vertex = add_vertex(*args)
        vertices << vertex
        yield vertex, i if block_given?
      end
      VertexSet.new(vertices)
    end
def add_n_vertices(n, *args)
      vertices = []
      n.times do |i|
        vertex = add_vertex(*args)
        vertices << vertex
        yield vertex, i if block_given?
      end
      VertexSet.new(vertices)
    end
Creates n vertices. _args_ can be module instances or hashes,
 which are all installed on vertices _v_ using  <tt>v.tag</tt>
 and <tt>v.add_marks</tt>, respectively. If a block is given,
 it is called after each vertex creation. The vertex is passed
 as first argument and the iteration index (from 0 to n-1) as
 second one.