When attempting to build a tunnel, if one is successful in the draw (i.e. having enough cards to cover the extras, or no extras revealed), do they still HAVE to build the tunnel? The rules were not clear about this, but I'm leaning towards no since it just says that only then can they build the tunnel. Last night, we had a player want to build the tunnel, then after he realized that he'd need to spend two extra cards, he opted not to...