SYSML2_ — StateAction::substates has an implied subsetting of exclusiveStates

According to the constraint checkStateUsageExclusiveStateSpecialization (in 8.3.17.6), a substate of a non-parallel owning state definition or usage must specialize StateAction::exclusiveStates (at least according to the description of the constraint; the OCL is wrong, see SYSML2_-183). However, the feature StateAction::substates actually meets the requirements for the application of checkStateUsageExclusiveSpecialization, which means that it has an implied specialization of exclusiveStates. But exclusiveStates already subsets substates (at least according to the library model in States.sysml; the feature exclusiveStates seems to be missing from subclause 9.2.10.2.1 StateAction), so this would mean that all substates are exclusiveStates, which is, of course, incorrect for the semantics of parallel states.

Make StateAction::substates an an action usage, not a state usage

This issue could be resolved by revising the checkStateUsageExclusiveStateSpecialization constraint so that it specifically excludes StateAction::substates. However, rather than making this feature a somewhat cumbersome special case, the feature could alternative simply be declared as an action usage rather than a state usage, so the constraint does not apply. This would not make any semantic difference, since the usage would still be typed by StateAction.

This resolution makes the later change, which only requires an update to the States library model.