Since the entire autopilot is heavily inspired by the [[ROS2 - NAV2 Library]] we also use [[Behaviour Trees]] to implement the autopilot.

All of our simple behaviors are located in the nav3_behavior_tree package under the plugins folder. There are [four types](https://www.behaviortree.dev/docs/learn-the-basics/main_concepts#create-custom-nodes-with-inheritance) of plugins that form the base of behavior trees as basic building blocks.: action, condition, control and decorators. 

1. Decide what kind of node you will be adding: we're assuming a control node for the sake of the example
2. write your h-file in *nav3_behavior_tree/include/nav3_behavior_tree/plugins/control*.
	1. As an example 
	2. Inherit from *BT::ControlNode*. (See round_robin_node.hpp as an example)
3. Write the cpp-file in *nav3_behavior_tree/plugins/control/round_robin_node.cpp*
	4. Make sure you use the `BT_REGISTER_NODES()` macro to register the node in the behavior tree factory.
4. Add the plugin to the *CMakeLists.txt: nav3_behavior_tree/CMakeLists.txt*
5. Write a test in the following file: *nav3_behavior_tree/test/plugins/control/test_round_robin_node.cpp*
6. Register the test in the following file: *nav3_behavior_tree/test/plugins/control/CMakeLists.txt*
	1. Make sure you test all cases that the node uses.
7. Add the new tree node to the library file: *nav3_behavior_tree/nav3_tree_nodes.xml*