[reviewing a 1969 revival of 'Of Thee I Sing'] The Gershwins may still be out on their own, ahead of the field, in the American musical theater. This is an extraordinarily advanced kind of musical comedy. They were here actually straining toward proper, or more likely, improper operetta and had the courage to to use arias, ensembles - even, as unlikely as it sounds, recitatives - and the musical aspect of the show is as fresh as tomorrow.