BTP issue maint-11 - proposed resolution

Title: Message

I propose we allow 
repetition of CANCELLED, using solution I proposed when submitting 
this:

 

Add additional state z2, entered on sending 
CANCELLED where this currently transits to z. If an imbound message is received, 
transit to a new query state (y3), which is exitted by resending CANCELLED, 
reverting to z2. Disruption (loss of volatile information) should cause a 
transition to existing z. (Thus the current behaviour can be treated as a 
disruption I.)