Stephen Sondheim has written 19 major musicals and, in his eighty-fourth year, is working on the twentieth. But there are also five extra shows created from offcuts of other musicals, including Putting It Together, a song cycle about marriage and infidelity (devised by Sondheim with frequent collaborator Julia McKenzie), that director Alastair Knights has staged at the St James Theatre in London.The show’s five characters are numbered rather than named – Man 1, Man 2 and Man 3 and Woman 1 and Woman 2 – and start out as two couples who are jumbled by a lover. As a domestic-marital drama, the show most resembles Sondheim’s Company and Follies. But it is a measure of the ingenuity of the evening that only five of the 32 featured ­numbers come from those two most o