What is an anchor?

[Upayavira]

I've just finished implementing my nested VariableResolver. No idea 
whether it works, though - that bit comes next.

Anyway, one thing in the existing PreparedVariableResolver is an Anchor 
variable. I've never come across these. Can someone give me an example 
of how they might work, so that I can make sure they still work with the 
new VariableResolver?

Thanks,

Upayavira