def remove_vertices(*vertices)
      vertices = to_vertices(*vertices).sort{|v1,v2| v2<=>v1}
      vertices.each do |vertex|
        remove_edges(vertex.in_edges+vertex.out_edges)
        @vertices.delete_at(vertex.index)
        vertex.index=-1
      end
      @vertices.each_with_index {|v,i| v.index=i}
      self
    end
def remove_vertices(*vertices)
      vertices = to_vertices(*vertices).sort{|v1,v2| v2<=>v1}
      vertices.each do |vertex|
        remove_edges(vertex.in_edges+vertex.out_edges)
        @vertices.delete_at(vertex.index)
        vertex.index=-1
      end
      @vertices.each_with_index {|v,i| v.index=i}
      self
    end
Removes all vertices returned by evaluating the _vertices_ selection
 expression.