Function clock 

pub fn clock() -> VectorClock

Get the current thread’s vector clock, or an empty clock if there is no current task.

Every BatchSemaphore operation calls this, so a Drop handler that touches a modelled primitive reaches it wherever the value is dropped: outside an execution, or while ExecutionState is borrowed. Panicking there happens inside a destructor, which can abort the process instead of reporting the failure under investigation. (While an execution is torn down, the task being torn down is the current task.)