# File lib/nanoc/base/directed_graph.rb, line 85
    def delete_edge(from, to)
      @from_graph[from] ||= Set.new
      @from_graph[from].delete(to)

      @to_graph[to] ||= Set.new
      @to_graph[to].delete(from)

      @roots.add(to) if @to_graph[to].empty?

      invalidate_caches
    end