[Greasemonkey] Colour-coding of visited links

[Edward Hibbert]

I have a script which colour-codes links.  This is interfering with the
native colour-coding of a link based on whether you've visited it before
- the native colour-coding is overwritten by mine.  
 
Is there any way for me to tell inside a script whether the link has
been visited, so that I can modify my colour-coding accordingly?