State subactions (entry, do, or exit) may reference existing actions.

Here is an example of such a reference:

part flashlight {

      perform ActionTree::produceDirectedLight;
      
      state on {
          // define the do action by referring to (actually subsetting)
          // the action above 'perfrom ActionTree::produceDirectedLight'
          do produceDirectedLight;
      }
}

Descision

We need to add new tools to create subactions of StateUsage and StateDefinition that allow to select existing ActionUsage to reference to.
New node tools should have a precondition that limits only one occurrence of each kind of subaction.

Semantic structure

Subactions should be linked to the State through a StateSubactionMembership which holds the kind to designate the actual subaction (Entry, Do, Exit).
This membership owns the PerformActionUsage of the subaction itself.
When the subaction is referencing another action, it owns a ReferenceSubsetting that holds the referenced action.

Impact on existing tools

Currently, there are tools New Entry Action, New Do Action, and New Exit Action on a StateUsage or StateDefinition that create ActionUsage elements instead of PerformActionUsage ones.
Therefore, these tools should be modified to be aligned with Pilot and syside implementations.

Nevertheless, the specification textual BNF says that when a sub action is an empty action, in this case it is an ActionUsage (empty):

StateActionUsage : ActionUsage =
      EmptyActionUsage ';'
      | StatePerformActionUsage
      | StateAcceptActionUsage
      | StateSendActionUsage
      | StateAssignmentActionUsage

EmptyActionUsage : ActionUsage =
      {}

      state on {
          // define an empty do action
          do ;
      }

To create such empty actions, we would need to add new tools to create empty ActionUsage