def display_prerequisites
      tasks.each do |t|
        puts "#{name} #{t.name}"
        t.prerequisites.each { |pre| puts "    #{pre}" }
      end
    end
def display_prerequisites
      tasks.each do |t|
        puts "#{name} #{t.name}"
        t.prerequisites.each { |pre| puts "    #{pre}" }
      end
    end
Display the tasks and prerequisites