concurrent.go
func (ui *ConcurrentUI) Log(message string) {
	ui.l.Lock()
	defer ui.l.Unlock()
	ui.UI.Log(message)
}
func (ui *ConcurrentUI) Log(message string) {
	ui.l.Lock()
	defer ui.l.Unlock()
	ui.UI.Log(message)
}
// Log calls UI.Log to write.
// This is a thread safe function.