For much of his life Pierre Boulez was preoccupied by the idea of a work being continually ‘in progress’. So many of his compositions exist in multiple versions that those which don’t – his early masterpiece Le marteau sans maître (1953–5), for example – are the exceptions that prove the rule. And while Messagesquisse (1976), which opens tonight’s concert, is one of those exceptions, it also represents the first link in a chain of pieces that stretches over several decades.