O/P redirection to file

Hello,

I'm redirecting my xsl o/p to a text file using 'redirect:open' and 
'redirect:write'. Default behaviour of these calls overwrite existing 
file. There are also options to append data to files. But I want to 
creat o/p file in case it does not exist, if file exists then i dont 
want to overwrite or modify it.
Is there any way to identify whether a text file exists, or create only 
if it does not exist?
Thanx & Regards,
Vikram.