How to replace function but still call it?

["Kenneth Rochel de Camargo Jr." <kenneth@...>]

Hello everyone.
I'm still a newbie in Lua, so if my questions are too simple I beg your 
patience, I've been looking for this one in the list archive and the documents, 
but I couldn't find an answer.
How can I redirect an intrinsic function and still call it?
I managed to override print without any problems. I'd like to override read and 
write too, but just for calls with no FILE arguments, which would use stdin and 
stdout for I/O. When such a parameter is present, I'd like to call the original 
implementation.
Thanks in advance,
Ken
Kenneth Rochel de Camargo, Jr. MD PhD
Associate Professor - Social Medicine Institute
Rio de Janeiro State U. - Brazil
e-mail: kenneth@uerj.br
http://usuarios.uninet.com.br/~kcamargo/