Throw RunTime error on invalid covering

        ArrayList<CanvasPoint> testCovering = testCanvas.generateCovering();

        System.out.println("Produced covering:");

        System.out.println(testCovering);

        System.out.println(" ... covering is "+(testCanvas.checkCovering(testCovering)?" valid!":" invalid"));

        if (testCanvas.checkCovering(testCovering)) {

            System.out.println(" ... covering is valid!");

        }

        else {

            throw new RuntimeException("Covering is invalid!");

        }

        // return results

        return new CoverResult(testCanvas, testCovering);