public class TestContraVariant {
    main(x, y) {
    	var y2 = m(x, y).snd();
		var z = null;
		y2 = z;
		return new Pair(x,z);
    }

    m(x, y) {
    	var x2 = main(x, y).fst();
	    var z = null;
		x2 = z;
        return new Pair(z, y);
    }
}