type SchedReqs struct {
	Thread    Constraint
	Proc      Constraint
	Goroutine Constraint
}

// Constraint represents a various presence requirements.
type Constraint uint8

const (
	MustNotHave Constraint = iota
	MayHave
	MustHave
)

// UserGoReqs is a common requirement among events that are running
// or are close to running user code.

	} else if reqs.Thread == event.MustNotHave && ctx.M != NoThread {
		return fmt.Errorf("expected no thread but had one")
	}

	// Check proc requirements.
	if reqs.Proc == event.MustHave && ctx.P == NoProc {
		return fmt.Errorf("expected a proc but didn't have one")
	} else if reqs.Proc == event.MustNotHave && ctx.P != NoProc {
		return fmt.Errorf("expected no proc but had one")
	}