as the question has been unanswered on answers.ros.org I hope here someone kindly comments
Tutorials refer to ros2 demos and to ros2 examples and both are actively maintained
both seem to rely on example_interfaces in /opt/ros/crystal/share/example_interfaces
Is that a historically grown thing ? if not when should I to look at which ?
Or in other words how would I extract the right files/infos to compose my own ros2 project ?