old_contents: 
  def hello():
      print 'Hello from the reactor loop!'
  
  from twisted.internet import reactor
  
  reactor.callWhenRunning(hello)
  
  reactor.run()
  
new_contents: 
  def hello():
      print 'Hello from the reactor loop!'
  
  from twisted.internet import reactor
  
  reactor.callWhenRunning(hello)
  
  print 'Starting the reactor.'
  reactor.run()
  
subject: Print before running reactor.