@Test
public void testIsNull() {

    ScelExpr expr = ScelParser.parse("a is null");
    assertEquals("a is null", expr.toString());
    expr = ScelParser.parse("a is not null");
    assertEquals("a is not null", expr.toString());
    expr = ScelParser.parse("a is null and b is not null");
    assertEquals("a is null and b is not null", expr.toString());
    expr = ScelParser.parse("(a is null) and (b is not null)");
    assertEquals("( a is null ) and ( b is not null )", expr.toString());
}