protected function filterColumn($col, $val, $op, $literal=false)
    {
        $valid_op = $this->validateOp($op);
        if ($valid_op === false) {
            throw new Exception('Invalid operator: ' . $op);
        }
        $this->filters[] = array(
            'left' => $col,
            'right' => $val,
            'op' => $valid_op,
            'rightIsLiteral' => $literal,
        );
    }