Able to or able not to?

I've been pondering again what is always a somewhat thorny issue: was Jesus able to sin or was he not able to sin? It arose again yesterday in a Patristics class in which we were examining the early work of Athanasius, de incarnatione. It is a question that is related to whether Jesus assumed a "fallen human nature" (which is sometimes suggested by the formula employed in Gregory of Nazianzen's First Letter to Cledonius that "the unassumed is the unhealed."