public function offsetSet($offset, $value)
    {
        if (is_null($offset)) {
            throw new \Exception('Must supply key to modify an associative array.');
        } else {
            if ($this->_isFillable($offset)) {
                $this->_args[$offset] = $value;
            } else {
                throw new \Exception('Cannot modify element \''.$offset.'\'. Not fillable.');
            }
        }
    }