public function setGroup( $groupName )
	{
		if( !$groupName )
			throw new InvalidArgumentException( 'No Group Name given.' );
		if( !file_exists( $this->fileName ) )
			throw new RuntimeException( 'File "'.$this->fileName.'" is not existing' );
		if( !$this->isWritable( $this->fileName ) )
			throw new RuntimeException( 'File "'.$this->fileName.'" is not writable' );
		if( !@chGrp( $this->fileName, $groupName ) )
			throw new RuntimeException( 'Only a superuser can change file group' );
	}