are you able to place this event in time in relation to Sondheim's very vocal disparagement of (some would say tantrum about) the Gershwin estate's handling of Porgy on Broadway in 2012? That might explain a lot. I try to judge people on their achievements, not their deficits. I don't know if or when this might have been a Sondheim demon. Rumors (all sorts), he had a few (to mix metaphors pretty badly), and that there is a backstory (or a few) with Feinstein seems palpable. Do we care? To me this says more about MF than SS.
