def add_vertex(*args)
      vertex = Digraph::Vertex.new(self, @vertices.length)
      apply_arg_conventions(vertex, args)
      @vertices << vertex
      vertex
    end
def add_vertex(*args)
      vertex = Digraph::Vertex.new(self, @vertices.length)
      apply_arg_conventions(vertex, args)
      @vertices << vertex
      vertex
    end
Adds a vertex. _args_ can be module instances or hashes,
 which are all installed on the vertex _v_ using  <tt>v.tag</tt>
 and <tt>v.add_marks</tt>, respectively.