[Greasemonkey] Overriding js functions and variables

[Alex]

Hi,

I would like to override a function in Gmail's javascript from my 
Greasemonkey script.  Can someone give me an idea of how to do this?  
Mihai Parparita's 'Gmail label colors' script 
(http://persistent.info/greasemonkey/gmail-label-colors.user.js) seems 
to have a routine in it for overriding functions, but I can't get his 
script to work.

Cheers,

Alex.