def getFirstLevel(self) :
		"""returns a list of couples (x1, x2) of all the first level indexed regions"""
		res = []
		if len(self.children) > 0 :
			for c in self.children:
				res.append((c.x1, c.x2)) 
		else :
			if self.x1 != None :
				res = [(self.x1, self.x2)]
			else :
				res = None
		return res