// and Next. Advance is called to try and advance an event and
// add completed events to the ordering. Next is used to pick
// off events in the ordering.
type ordering struct {
	gStates     map[GoID]*gState
	pStates     map[ProcID]*pState // TODO: The keys are dense, so this can be a slice.
	mStates     map[ThreadID]*mState
	activeTasks map[TaskID]taskState
	gcSeq       uint64
	gcState     gcState
	initialGen  uint64