You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: add_vertex.rb
    path of file: ./repos/artichoke/artichoke-backend/vendor/ruby/lib/bundler/vendor/molinillo/lib/molinillo/dependency_graph
    
    the code of the file until where you have to start completion: # frozen_string_literal: true

require_relative 'action'
module Bundler::Molinillo
  class DependencyGraph
    # @!visibility private
    # (see D
    
      def down(graph)
        if defined?(@existing_payload)
          vertex = graph.vertices[name]
          vertex.payload = @existing_payload
          vertex.root = @existing_root
        else
          graph.vertices.delete(name)
        end