I'm looking through the rules trying to figure what alliances are legal, and all I keep finding is detachment restrictions that say that units in the detachment must share a faction keyword, but I'm not seeing a similar restriction on the army as a whole. Am I missing something or could I potentially do an Eldar / Necron / Chaos alliance (or something equally crazy) and it still be a legal battle-forged army as long as I have them organized into proper detachments?