public QueryBuilders.QueryBuilder convert(Expr conjunct) {
        return visit(conjunct);
    }
@Test
    public void testTranslateIsNullPredicate() {
        SlotRef kSlotRef = mockSlotRef("k", Type.BOOLEAN);
        IsNullPredicate isNullPredicate = new IsNullPredicate(kSlotRef, false);
        IsNullPredicate isNotNullPredicate = new IsNullPredicate(kSlotRef, true);
        Assert.assertEquals("{\"bool\":{\"must_not\":{\"exists\":{\"field\":\"k\"}}}}",
                queryConverter.convert(isNullPredicate).toString());
        Assert.assertEquals("{\"exists\":{\"field\":\"k\"}}",
                queryConverter.convert(isNotNullPredicate).toString());
    }