def summary
      @lock.synchronize do
        puts "jobs: #{@input_queue.size}"
        puts "workers: #{@workers.size}"
        @workers.each do |w|
          puts "  " + w.thread.inspect
        end
      end
    end
def summary
      @lock.synchronize do
        puts "jobs: #{@input_queue.size}"
        puts "workers: #{@workers.size}"
        @workers.each do |w|
          puts "  " + w.thread.inspect
        end
      end
    end
For debugging