Function is_exempt 

pub fn is_exempt(name: &str) -> bool

Names no contract has to declare: use_sim_time and start_type_description_service, which rclcpp declares on every node, and qos_overrides.*, which is derived from the contract itself. These are the names play_launch exempts when it holds a launch file to the same contract, so the launch check and this one agree about what a node may carry.