protected static int determineCase (Node<Coordinate2D> n, double t) {
		assert n != null : "n != null";
		assert n.isLeaf : "n.isLeaf";

		boolean leftAbove = true;//fake left corner (at y=-infty)
		boolean rightAbove = false;//fake right corner (at y=-infty)
		
		if ( (n.left != null) && computeSlope(n.left, n) < t  ) {
			leftAbove = false;
		}
		
		if ( (n.right != null) && computeSlope(n, n.right) > t  ) {
			rightAbove = true;
		}
		
		if (leftAbove && rightAbove) {
			return -1;
		}
		else if (!leftAbove && !rightAbove) {
			return +1;
		}
		else {
			assert leftAbove && !rightAbove : "concave";
			
			//corner case for single-node-SubHull also comes here
			return 0;
		}
	}