public function setOwner( $userName )
	{
		if( !$userName )
			throw new InvalidArgumentException( 'No User Name given.' );
		if( !file_exists( $this->fileName ) )
			throw new RuntimeException( 'File "'.$this->fileName.'" is not existing' );
#		if( !$this->isOwner() )
#			throw new RuntimeException( 'File "'.$this->fileName.'" is not owned by current user' );
		if( !$this->isWritable( $this->fileName ) )
			throw new RuntimeException( 'File "'.$this->fileName.'" is not writable' );
		if( !@chOwn( $this->fileName, $userName ) )
			throw new RuntimeException( 'Only a superuser can change file owner' );
	}