It’s 1771, you’re in Milan, and your 14-year-old genius son has just premièred his new opera. How do you reward him? What would be a fun family excursion in an era before multiplexes or theme parks? Leopold Mozart knew just the ticket. ‘I saw four rascals hanged here on the Piazza del Duomo,’ wrote young Wolfgang back to his sister Maria Anna (‘Nannerl’), excitedly.