# File lib/nanoc3/base/dependency_tracker.rb, line 160
    def store_graph
      FileUtils.mkdir_p(File.dirname(self.filename))
      store = PStore.new(self.filename)
      store.transaction do
        store[:version]  = STORE_VERSION
        store[:vertices] = @graph.vertices.map { |i| i && i.identifier }
        store[:edges]    = @graph.edges
      end
    end