public static function sum( $arr, $key = null ) 
	{	
		if( !is_array( $arr ) ) 
		{
			throw new \InvalidArgumentException('CCArr::sum - first argument has to be an array.');
		}

		$sum = 0;

		if ( is_string( $key ) && CCArr::is_multi( $arr ) ) 
		{
			$arr = CCArr::pick( $key, $arr );
		}

		foreach ( $arr as $item ) 
		{
			if ( is_numeric( $item ) ) 
			{
				$sum += $item;	
			}
		}

		return $sum;
	}