old_contents: # BEGIN ALL
  #!/usr/bin/env python
  import rospy
  from geometry_msgs.msg import Twist
  
  cmd_vel_pub = rospy.Publisher('cmd_vel', Twist, queue_size=1) #<1>
  rospy.init_node('red_light_green_light')
  
  red_light_twist = Twist() #<2>
  green_light_twist = Twist()
  green_light_twist.linear.x = 0.5 #<3>
  
  driving_forward = False
  light_change_time = rospy.Time.now()
  rate = rospy.Rate(10)
  
  while not rospy.is_shutdown():
    if driving_forward:
      cmd_vel_pub.publish(green_light_twist) #<4>
    else:
      cmd_vel_pub.publish(red_light_twist)
    # BEGIN PART_1
    if light_change_time > rospy.Time.now(): #<5>
      driving_forward = not driving_forward
      light_change_time = rospy.Time.now() + rospy.Duration(3)
    # END PART_1
    rate.sleep() #<6>
  # END ALL
  
new_contents: # BEGIN ALL
  #!/usr/bin/env python
  import rospy
  from geometry_msgs.msg import Twist
  
  cmd_vel_pub = rospy.Publisher('cmd_vel', Twist, queue_size=1) #<1>
  rospy.init_node('red_light_green_light')
  
  red_light_twist = Twist() #<2>
  green_light_twist = Twist()
  green_light_twist.linear.x = 0.5 #<3>
  
  driving_forward = False
  light_change_time = rospy.Time.now()
  rate = rospy.Rate(10)
  
  while not rospy.is_shutdown():
    if driving_forward:
      cmd_vel_pub.publish(green_light_twist) #<4>
    else:
      cmd_vel_pub.publish(red_light_twist)
    # BEGIN PART_1
    if rospy.Time.now() > light_change_time: #<5>
      driving_forward = not driving_forward
      light_change_time = rospy.Time.now() + rospy.Duration(3)
    # END PART_1
    rate.sleep() #<6>
  # END ALL
  
subject: fix sign error