= R"(
<root BTCPP_format="4">
<BehaviorTree ID="MainTree">
<Sequence>
<Fallback>
<AlwaysFailure name="failing_action"/>
<SubTree ID="SubTreeA" name="mysub"/>
</Fallback>
<AlwaysSuccess name="last_action"/>
</Sequence>
</BehaviorTree>
<BehaviorTree ID="SubTreeA">
<Sequence>
<AlwaysSuccess name="action_subA"/>
<SubTree ID="SubTreeB" name="sub_nested"/>
<SubTree ID="SubTreeB" />
</Sequence>
</BehaviorTree>
<BehaviorTree ID="SubTreeB">
<AlwaysSuccess name="action_subB"/>
</BehaviorTree>
</root>
)"
Show the use of the TreeObserver.