In the year following its 1787 Prague premiere, Don Giovanni was performed in Vienna. Everyone, according to the well-known account by Da Ponte, thought something was wrong with it. In response, Mozart made changes, producing a Vienna 'version' of the opera, cutting two of the original arias but inserting three newly-composed pieces. The dilemma faced by musicians and scholars ever since has been whether to preserve the opera in these two 'authentic'...