As the killing of civilians escalates in Syria, diplomats at the U.N. Security Council are grappling with a vexing issue: If they demand that Syrian President Bashar al-Assad step down, and Assad thumbs his nose at them, what then?At issue is a resolution backed by the United States, Britain, France, the Arab League and others, that insists that al-Assad not only stop the violence against his people, but give up power within 15 days -- or else.