Arjun and I have continued to work on our project on fiscal and monetary policy, which develops the simple -- but strangely overlooked  -- point that both the level of output and the trajectory of the public debt-GDP ratio are jointly determined by both the government budget balance and the interest rate set by the monetary authority. (An early stage in our thinking on this was the subject of a post on this blog last year.) Part of our argument is that the fiscal space metaphor is backward -- that the case for countercyclical fiscal policy gets stronger, not weaker, when debt ratios are already high. I'm hoping there will be a working paper version of this soon. But in the meantime, the work is getting presented at various places -- by me at the Eastern Economic Association this past spring, by both of us at the International Economics Association in June, by Arjun at an OECD conference in Rome earlier this week, and by me at the University of Missouri at Kansas City tomorrow. If you're interested, here are our current slides.
 This paper by Michael Woodford, which I haven't yet had time to read properly, seems to have a similar starting point but ends up somewhere quite different.