Previously I had described a "Group" or "Group of Actions" as a separate entity from "Action", but since then I think we can just have a special Action that yields other actions and optionally does things like pushes/pops the launch configurations and set/unset specific launch configurations.

I believe the action I've described is capable of fulfilling the use cases I originally imagined for groups:

• push containing nodes into a namespace
  • would be done by modifying the ros_namespace LaunchConfiguration that affects only the actions it yields
    • see:

      launch/launch_ros/launch_ros/actions/node.py

      Lines 80 to 81 in b78d87f

      	If relative, the namespace in the 'ros_namespace' LaunchConfiguration
      	will be prepended to the given relative node namespace.

• conditionally include other launch descriptions or actions
  • done with proposed Condition concept which would work on any Action, including this one
    • see: [launch] add concept of Conditions #105
• scope arbitrary launch configurations
  • done by using the LaunchContext to push and pop them
    • see:

      launch/launch/launch/launch_context.py

      Lines 146 to 157 in b78d87f

      	def _push_launch_configurations(self):
      	self.__launch_configurations_stack.append(dict(self.__launch_configurations))
      	def _pop_launch_configurations(self):
      	if not self.__launch_configurations_stack:
      	raise RuntimeError('launch_configurations stack unexpectedly empty')
      	self.__launch_configurations = self.__launch_configurations_stack.pop()
      	@property
      	def launch_configurations(self) -> Dict[Text, Text]:
      	"""Getter for launch_configurations dictionary."""
      	return self.__launch_configurations

We should also update the design document (ros2/design#163).