Assertions can be globally defined outside the scope of any state, and individually referenced from one or more states using an assertionRef element by ID or name. See the Assertions section of the Design document for a full explanation.
This improvement is to:
Introduce globally-defined assertions to the model
Add the optional name attribute to assertions - only for globally-defined assertions
Add support for "single" type assertionRef's to states
Update the assertion logic to include referenced assertions when verifying a state,