public static void addSelectedTray(Tray tray) {
		if (tray == null
				|| StorageUnitDisplayPanel.selectedTrays.contains(tray)) return;
		if (!StorageUnitDisplayPanel.isSelectable(tray)) return;
		if (StorageUnitDisplayPanel.selectedTrays.add(tray)) StorageUnitDisplayPanel
				.triggerTraySelectionChanged();
	}