# File lib/nanoc/base/directed_graph.rb, line 60
    def add_edge(from, to)
      add_vertex(from)
      add_vertex(to)

      @from_graph[from] ||= Set.new
      @from_graph[from] << to

      @to_graph[to] ||= Set.new
      @to_graph[to] << from

      @roots.delete(to)

      invalidate_caches
    end