String literals have only one instance?

[Stewart Gordon]

Jonathan Davis wrote:
<snip>
> You can always check with the is operator though. If it reports true,
> then the two strings have the same instance. If it reports false, then
> they don't.

I can't see how testing each string literal to see if it's the same 
instance as another can work.

The OP's point is: Are identical string literals *guaranteed* to be the 
same instance?  Regardless of implementation?  Regardless of whether 
they're next to each other, in different modules or anything in between? 
  Regardless of the phase of the moon?

Stewart.