tc, ok := getTraceContext(ctx)
	if !ok {
		return ctx
	}
	return context.WithValue(ctx, traceKey{}, traceContext{tc.t, tc.t.getNextTID()})
}

// Flow marks a flow indicating that the 'to' span depends on the 'from' span.
// Flow should be called while the 'to' span is in progress.
func Flow(ctx context.Context, from *Span, to *Span) {
	tc, ok := getTraceContext(ctx)
	if !ok || from == nil || to == nil {