Mona Pirnot’s new play, I’m Assuming You Know David Greenspan, concerns the hardscrabble existence of aspiring playwrights and the passion that keeps them writing for an industry in which, as playwright Robert Anderson ostensibly said, it’s possible to make a killing but never a living. David Greenspan is the very model of a theater artist who has persevered despite dire fiscal odds. Greenspan is pretty well-known Off-Broadway and, especially, Off-Off Broadway, but he’s certainly not a household name.