In 2010, at the height of the foreclosure crisis, the federal government watched nervously as hundreds of thousands of families lost their homes. Empty houses blighted neighborhoods, their shades drawn, their yards overgrown. Without some kind of intervention, federal officials worried, the housing market would continue in its free fall, prices would keep dropping for existing homeowners, and the economic recovery, already tenuous, would be imperiled.But who would fill these empty homes? Few Americans were in a buying mood, and for those who were, mortgages were harder to come by than they had been before the crash.