public function where($where, $op=null, $value=null)
    {
        if ($where) {
            if (is_array($where)) {
                $this->set('where', $where);
            } elseif (!is_null($op)) {
                if (!is_null($value)) {
                    $value = [$op, $value];
                } else {
                    $value  = $op;
                }
                
                $this->set('where', [$where => $value]);
            } else {
                $this->set('where', $where);
            }
        }
        
        return $this;
    }